Every PSP I own must have the following games ready to go at any time:
-Lumines
-Rock Band Unplugged
-Patapon 2
-WipEout Pulse
-Final Fantasy: Crystal Defenders (gorgeous small tower defense game, but with enough strategy behind it for hours of play)
It's a pretty great game. Really good music and one of those gameplay loops that is easy to pick up and difficult to master. If you like Tetris and the like, Lumines is worth your attention.