A friend is travelling to Berlin. got any recommendations?

one word. Oktoberfest.

If he's into high quality clothing and shoes I would be able to give you a list of shops worth checking out, but other than that I think I am not able to help.
Maybe check for interesting art exhibitions that are on at the time.

he should defenitly take a free guide trough the reichstagsgebäude (the german government building)
https://visite.bundestag.de/BAPWeb/pages/createBookingRequest.jsf?lang=en

also walk along the old berlin wall which was painted by artists -> https://upload.wikimedia.org/wikipedia/commons/1/10/Gallery.jpg
visit checkpoint charlie and some museums (like DDR-museum, which is more interessting than it sounds) -> http://www.ddr-museum.de/en/museum

visit the dome at the evening and walk on it's top -> http://marius-klemm.de/wp-content/uploads/2013/04/Berliner-Dom-HDR.jpg
he can also go down to the tombs -> http://www.berlinerdom.de/images/stories/dom_innen/gruft/sarkophag_philipp_wilhelm.jpg

see the brandenburger tor, near that there is also a Madame Tussauds in berlin

walk in the holocaust memorial ->http://blogs.cnnmexico.com/en-primer-plano/wp-content/uploads/2010/07/memorial_berlin1.jpg
visit the jewish cathedral

there is a huge botanic garden in berlin, where you can walk your feed off. you have a diversity of plants outside, and tropical plants inside -> http://www.bgbm.org/en/garden/survey-garden

a guide in an old german bunker -> http://berliner-unterwelten.de/guided-tours.3.1.html

he should have visited a käthe wohlfahrt shop -> http://wohlfahrt.com/en/christmas-stores/berlin
not for shopping, just for looking all the christmas stuff which is open all over the year

trip advisor should help finding places worth to see too

he shouldn't waste his time on shopping tours.

I have lived in Berlin for about 4 months, i don't know much but my recommendations would be the most obvious.
My favourite attraction of all is the Berliner Fernsehturm, it's a giant TV tower that you can go to the very top and watch the whole city from there, there is also a restaurant in there but it's almost always full and you probably need a reservation.

Not too far from there is the Berliner Dom, a really old german church that you can visit and it's a pretty interesting place overall.

As a Berliner myself I can recommend alot of things. "Fernsehturm" at the Alexanderplatz for example. Also when he is at the alexanderplatz he can visit the Berlin Dungeon. It's a little history of Berlin (two different tours, one in english and one in german) with a mix of horror and entertainment. Potzdamerplatz (close the Brandenburgertor) is also worth checking out.

Haven't been there myself yet but it kinda sounds interesting - what about the video game museum?

Prostitutes. It's something interesting to do ;)

(To be serious...I don't know. She could visit my aunt.)

Tell your friend to go to Brandenburger Tor..After reflecting there (or not) for a few minutes, reach for one of the side pillars. Somewhere there is the entrance to the Raum der Stille, or in world lingo: Room of Silence..
It is a small sound-isolated room where people irrelevant of colour, beliefs, religion and other such causes of war, can go in and enjoy the silence. Unless you have tinnitus in which case it gets very loud in there.
No pictures, no texting, no phones, no chat.. Just shut up for a few minutes and enjoy being the only person in the world..

Find hitlers grave and pee on it.

Never been there, but if he/she is interested in historical touristy stuff to see, this guy makes good videos about different cities around the world.

Visit Berlin - The Top 10 Sites in Berlin, Germany

Visit Berlin - 5 Things You Will Love & Hate about Berlin, Germany

I went up to Sachsenhausen. Don't, if you want to remain in a good mood.

Gift him Shadowrun: Dragonfall, it takes place in Berlin. (Just after some nuclear wars, total cyborgization and the awakening of magic)

visit the video game museum!!!! i loved it.

especially the arcade where you get electrocuted & beaten if you lose a point.

my fav restaurant: http://www.umspannwerk-ost.de/de/restaurant/

Okay, this is gonna be a long one.

First it depends on how long he is going to be there, because certain things rank higher than the others.
I will +1 the Reichstag tour, but he has to preregister for it, because ... well because. So it can't be a last minute thing he wants to do.

I would definitely recommend Alexanderplatz and the Fernsehturm (Pay the money go up and see the sights in the tower)

Oh that, if he likes Museums, he should check out Museuminsel (Museum Island) there are five of the most famous ones there. If he likes Museums a lot, ask me for more recommendations because Berlin has tons of Museums and depending on what he is into, depends on where he should go.

If he has some free time, he can check out the Topographie des Terrors which is a free museum about World War II.

I would check out a Flea Market (Flohmarkt) and the one I would suggest if he wants to be overwhelmed but see a ton of stuff (just know it's overpriced) is at Mauerpark on Sundays. Further, he will see a ton of Germans hanging out and if he's lucky they will be doing Karaoke in the park. Probably playing Soccer and other things as well. If he wants to know of some smaller Flea Markets that are fun, let me know.

Get off at Nordbahnhof station and travel to the northeast on Bernauer Strasse. It is a monument to the Wall itself and will show you the different ways the guard towers worked and memorials to the people who lost their lives. There is also a bit about the church that was demolished so they could build the wall. :/

If he wants to see cool shopping centers, then Potsdamnerplatz is definitely the place to go, and they have Movies in English at the Sony Center.

I would recommend going to Gendarmenmarkt off the U-Bahn and see the two churches and the symphony house.

Everyone should of course see Brandenburger Tor as well, BUT, make sure you schedule one of the free walking tours that picks up outside Starbucks (They will ask for a donation at the end of the tour, you can find it outline, free walking tour... takes about 2-3 hours very worthwhile). Random fact while he is there, there is a hotel there the Adlon and that's where the Micheal Jackson scandal about holding a baby over the edge of the balcony happened.

Depending on how much time he has, Tiergarten (The huge park in the center) is fun to walk through and there are a ton of hidden monuments. Also, the Zoo can be fun, but if you're not a zoo person, skip it.

Get off at Mehringdamn and walk south, there is what is questionably (people have different opinions) one of the best Doner Kebap places in the city. Further, he'll be in the Kreuzberg district which has tons of bars, clubs, comedy clubs, swinger clubs?! and more to chose from.

If he wants to see a good Farmer's market Get off at Kotbusser Tor and then walk south on Kotbusser strasse. It will be on the left hand side after crossing the bridge. I believe it's Tuesdays and fridays, but I dont' remember, it's been awhile, if he looks it up or asks when he is there someone should be able to tell him. There is so much food from different places to try there.

Get off at Warshauer strasse. There are tons of little clubs that cater to not having to be so cool / hipstery, etc and speak English in that area. Just look for the masses and follow them at night. Don't start your evening till around 11pm (Though Berliners might way till 1am.. on a weekend)

Get off at Sophie-Charlotte Platz and make his a little east way to Schloss Strasse and head north. He will run directly into Schloss Charlottenburg (A nice castle/palace.) Behind it is a large park and a lake/pond type thing with ducks. You can take a tour inside for some nominal fee, but if he really likes castles, there is a two/three? day pass that will allow you into many of the castles in Berlin AND Potsdam for a slightly larger fee. Oh, and don't take pictures while on the tours WARNING unless you pay the small additional fee for photography rights.

Get off at Oranienburger Strasse in the early evening and just walk around, he'll see some interesting things. Same could be said for getting off at Nollendorfplatz and heading east along Bulowstrasse.

Depending on how much time he has and how interested he is, he can take a trip to the Olympic Stadium.. With Nazi symbols and all. Mind was blown. It's easy to find, there are two U-Bahns that go out that way and the stop is named, Olympiastadion. There are guided tours or self walking tours.

This requires a bit more exploration because I think he'll need buses and some walking, but Gedenkstätte Berlin-Hohenschönhausen is a Gestapo Prison in Berlin that he can take tours of.

If he has time on a weekend (Supposedly weekdays too, but I always went on weekdays) and he is comfortable with long walks and being VERY EXPLORATORY he could head up to Teufelsburg. The largest mountain in the Berlin area and it's man made. It is the home of the abandoned US Military spy base that after the wall came down was no longer necessary. If he does his research (bring water) and manages to find it, (it's not too hard) they now offer paid guided tours. This didn't used to be the case, but some people and the city government screwed it up for the rest of us. There is AMAZING graffiti there that if you were to take a picture of it and focus on it, excluding the other parts of the wall, you'd think it was a painting. HUGE HUGE art pieces. But, this is very out of the way.

Get off at Berlin Ostbahnhoff and walk southwest out of the station to Stralauer platz. Walk northwest along that street till he can make a left and cross the bridge over the river Spree. As he is crossing the river, be on the west side of the bridge and you'll find a dirt path heading down to the south side of the river bank. It will take you down to the back entrance of the old Ice Factory. It doesn't matter if you go here, the police don't bother anyone so long as they aren't doing anything stupid. You can see the place and there is a lot of graffiti in there as well.

If he likes plants, get off at Berlin-Marzahn. But then he'll need to find the correct bus to take him to Garten der Welt (Gardens of the World). Very cool place, so many different plants and even bits of architecture. (However, if his German isn't flawless, only do this in the daytime. Do not stay in this area late at night as a foreigner.)

Another trip that is a bit further out, is Pfaueninsel (Peacock Island). There is a mini castle there as well as tons of peacocks. Pretty interesting place to wander around. To get there, he needs to get off at Wannsee and find the correct bus to take him to the Ferry. The price I think is something like 5 Euros.

Take the S-Bahn out to Potsdam Hauptbahnhof. Another trip that is out in this direction, is to Schloss Sanssouci. Another palace... very famous. Huge gardens, amazing. Look it up before you decide to go or not go. There is so much to see there and you can spend at least half a day out there. There are multiple castles, another Brandenburger Tor (I know... I know...) and a really interesting church Friedenskirche. You don't really need to take a bus to visit this area, but you can if you wish. Walking with a map and perhaps a phone to be sure of where you are should be sufficient. But if you want to, you'd exit the south side of the station and most of the buses pick up there. Otherwise walk east and north over the bridge and you'll see more museums and interesting things. I would recommend walking along Friedrich-Ebert Strasse till he hits brandenburger strasse and making a left. That will lead to Brandenburger Tor. From there cross the square across the street from the Tor and head out the north western side. That street will take you directly to the Friedenskirche and the beginning of the Park Sanssouci which houses the palace, the fountains, sculptures and the Neues Palais (New Palace). After all this, you can easily find buses back to the Train Station and back to Berlin Proper. However if you have the time, you can stop in at Pub a la Pub on Breite Strasse (It's close to the museum of film in the beginning) and grab a few cheap beers.

There is a lot more to see in Berlin and Potsdam, but it really depends on what kind of time he has and how long he'll be there and what is interesting to him. Anyways, I think I've been writing this for over an hour now. Looking up information, etc. So if he needs/wants anymore information, let me know. I need a break. If he wants to go to clubs, if he wants to do other things and can be more specific, I can give more specific and other details. I'm sure I could come up with more... but... again... writing too long.

I lived in Berlin for 13.5 months, hence the details and information

I'll just make a short one:
Go on any of the Sandeman's new europe tours, they're pretty awesome, and there's even some for 'free'!

EDIT: This is my personal opinion, I don't know if this is considered some kind of product placement and if I shouldn't mention it though?

If he is not so much into games maybe this is more of interest.

Never been in Berlin, my sister has been though and she said it's a little too grey for her taste ;)
But he could listen to some good german music like these (it's rap but good one not that stupid gangster shit):
clicky1
clicky2
clicky3 (this song is about Berlin :P)

I was a day ago in berlin, i can recommend the Hotel Schildow, the Pizzeria in Schildow, the alexander place, brandenburger door, the Berlin TV tower and the Dong Xuán Centre :)

He definitely should visit the abandoned theme park called "Spreepark". However it's kind of illegal to go in there without a guided tour, but who cares.

http://www.abandonedberlin.com/2014/05/spreepark-abandoned-amusement-park.html
http://www.abandonedberlin.com/2010/04/spreepark-and-how-to-get-in.html