Play games like Tomb Raider I got over 700 points for little over a weekends work (I quite enjoyed the game too).
John Madden is another easy one, I got all 1000 pts in a week after borrowing it and Fight Night from a friend.
I've now just gone back to playing Oblivion, done a few more missions on the main quest (that's another 100 pts, thank you), and hopefully I'll get a few more achievements in either the main or fighters guild quests before too long (if I don't get too side tracked in other quests - currently trying to do the destruction master trainer quests "find 20 bear skins" I'm up to 18 but the last 2 bears are avoiding me (wish I hadn't sold any before I knew I needed them
)