Easy Games for 100% Achievements

The Plan... just one achievment and you are through the game within 5 min..
And it's free to play xD

Costume Quest
Stacking
Thomas Was Alone

bad bots

Home Sheep Home 2

They Bleed Pixels
Super Meat Boy
Dungeons of Dredmore

Thomas Was Alone

In Lococycle it's very easy to get all the achievements, as the game is not too hard and you basically just have to beat the story once.

IIRC my first 100% game was Call of Duty: Black Ops II Multiplayer ;)
Followed by Free To Play and The Plan.

LEGO games (LOTR, Marvel Super Heroes, Movie, Hobbit) are usually easy to obtain, but you need a lot of time to get them (like Get all red bricks or collect 10,000,000,000 studs)

Audiosurf
Pac Man Championship Edition DX+
NiGHTS Into Dreams
Sugar Cube: Bittersweet Factory
Scribblenauts Unlimited
Shadow Warrior Classic Redux (If you use cheats in game.)
Snuggle Truck
Blood Knights
Tobe's Vertical Adventure
Rush Bros
Ultionus: A Tale of Petty Revenge
Zack Zero
Superfrog HD

And mostly games you enjoy really much, I completed these 100% easily:
Wolfenstein: The New Order
Sleeping Dogs
Dead Island

Half-Life 2 and episodes can be done relatively easy
Deadly 30 - warning, some of them require you to not die/quit, like the Getting Paid one
Lara Croft and the Guardian of Light - most of them are easy but 1 is hard, requires you to collect everything which takes a while

The Plan

Garry's Mod

To the moon

The 39 Steps. You get 100% simply by playing the whole game. You might have to find all the hot spots maybe, but it's hard to miss them. :)

Hero of the kingdom.

no one mentioned the Blackwell series so here I go...
the blackwell series are pretty easy achievable

Always Sometimes Monsters

To the moon - just one achievement uncloked when finishing the game.
A New Beginning - Final Cut - just playing the game. In 3-4 places you have to play diffrent. Just loading save, and then continue.

Thomas was alone was pretty easy , achievements for each chapter and achievement to grab the "secret" 2 per chapter , and secret was out in the open impossible to miss o:

Few easy games i played + estimated time to get every achievements
Defy Gravity time : 3 hours
Lunnye Devitsy time : 4 hours
Mechanic Escape time : 7 hours
Constant C time : 8 hours
Fez time : 11 hours
Dust: An Elysian Tail time : 18 hours
Kingdom Rush time : 20 hours

Brothers - A Tale of Two Sons?

The 39 Steps. Easiest 100% ever. ;)

Estranged: Act I

The Walking Dead. Just play the game and you'll get a 100% cheevos at the end. Oh, and also, DLC Quest. The achievements are pretty easy to get except for the last 2 hidden cheevos (speedrun)