Here is a list of easy games for achievements.
Games finishable in a day (a week for those who have a life):Avatar The Last Airbender: The Burning EarthThis is the easiest game ever to get achievements, you can get them in the first five minutes of the game.
Peter Jackson's King Kong:King
Kong was one of the original easy games for achievements. All you need
to do to get all the achievements is finish the game once, you can even
use an invinceabillity cheat and it still works.
NBA 2K6:This game can be frustrating to get one or two of the achievements but is basicly extremely easy.
Viva Piņata: Party AnimalsI got 4 of the achievements in the first race, and all of them in 2 days.
EA SPORTS Fight Night Round 3All you need to do is win every tournament in career mode

.
Games finishable in a week or less (2-4 weeks for those who don't play obsessively):Viva PiņataThe
achievements on this may take some time because half of them are
secrets and others just take time too do but other then that they are
pretty straight fowards.
OblivionMost of the
achievements on this just require finishing the main stories or the
guilds, you do have to do side-quests to train though. To get all the
achievements you need to buy the shivering isles off
xbox live or buy
the game of the year edition.
Dynasty Warriors: GundamThe
achievements on this are simple, most of them are just to finish the
game with every character, others you will just get by finishing the
game with every character.
Phantasy Star UniverseAll
you need to do to get the achievements on this game is defeat the
bosses, which you will do by finishing the main story line.