Co-op Games!

F1 Race Stars

I don't think you should play games that contain elements of friendly fire i.e. Portal, possibly Lego as that might create a bit of relationship tension. However, I could be wrong, as I don't know the temperament of your GF and whether she would get pissed off by a bit of trolling about. Other than that I don't really have much for you.

you should go with LEGE series, cuz they re all relaxing, really easy to play and they all have great humor in my opinion

Battleblock Theater and Castle Crashers.

facebook casual games ;)

Dungeonland, has a large free demo version.

have hard good things about lara croft and the guardian of light in this regard.

Tiny Brains - http://store.steampowered.com/app/253690/
Gameplay Video
Looks like fun in coop. :)

Portal 2

Sonic & All-Stars Racing Transformed. You'll have a blast!

ObsCure

Thank god no one said Borderlands....overrated piece of expensive crap from a full-of-shit company.

FEAR 3 is nice in coop but dont know if you can play it on pc so. have only played on ps3

Rayman Origins! Legends probably too but I never played it.

i was in the same boat as u a few weeks ago; our GFs might have or not the same taste in games, but these are my results:
-likes mostly hack/slash, beatiful graphics and "shared screen" camera;
-dislikes the wasting time of reading skills/text from npc; not so good graphics;

some examples (+ = likes, - = dislikes):

• Dungeon Siege 3 (likes mostly the killing part, selling and chat is boring);
• Sacred 3(same as above);
• How to Survive(same as above);
• Minecraft(she is into building type of games);
• Dynasty Warriors 8(doesnt like the splitscreen camera though);
• castle crashers (dieing too much, graphics +-)
• Trine 2 ( not too much killing, puzzles get a bit in the way)
• most of the pixel games out there.

hope to have helped u somehow

Megabyte Punch,
Expendabros,

Hero Siege or Hammerwatch ;)

Monaco

XOXO

Spelunky, the new Binding of Isaac , Brothers : A tale of Two Sons , PID , Monaco , Guacamelee, Dead Rising 2 or 3 .

I'd go with Guacamelee, pretty simple controls imho, easy to learn.

Metal Slug, Trine 2

Guacamelee is awesome