Well, you already mentioned Peace Walker so you're on the right track

If you want fun racing games, I highly recommend Motorstorm Arctic Edge and Gran Turismo.

If you were ever into Twisted Metal, Head On is probably the best in the series. It's everything that made the first two games great, made by the same original team, and is just a very responsive and fun action game.

Hot Shots Tennis is a very addictive and charming game in the vein of the Mario sports games.

My favorite games on the PSP are Valkyria Chronicles II and Tactics Ogre. Both are evolutions of the SRPG formula, VC more overtly and TO more subtly. It's hard going back to traditional SRPGs without the Wheel of Fortune mechanic.