if you're looking for a cheap fun game now, I'd suggest Gears of War, Assassin's Creed, Rainbow Six: Vegas (if you like tactical games) or something else which tickles your fancy. If you can wait until Christmas or so, or maybe next year to spend more money, you could try getting any of those new games, like Call of Duty, Battlefield 3, Deus Ex or the new Naruto game which is set to come out soon.
I'm no expert on games when it comes to the Xbox, but you could try maybe other games, like Fable 2 or 3 (I personally prefer 2), or anything, really. Most games shops have a refund guarantee for a week's purchase, so if you don't like it you can return it and get your money back. ^_^