So you have no PSN games?
You must get Flower, it is a work of art, a real masterpiece.
Critter Crunch is a brilliant puzzle game, which is also really cheap.
I would also recommend Blue Toad Murder Files, if you can play it with friends or family (the replay-value is basically non-existant, so you have to play it with a group the first time. It only requires one controller though). I don't own them (yet) however, so...
I've heard great things about Braid as well, I doubt you would go wrong with that.
Other than that, I suggest checking out some demoes. If you like tower defense, I can recommend that you try PixelJunk Monsters and Comet Crash. I have only tried the Comet Crash demo, but PJM is a blast to play in coop.