My PSP has been shelved, along with my DS and 360 (I don't travel much anymore, and I picked up MW:2 for the PS3, everything else that is on the PC), so I can't really give a recent recommendation.
That being said, I still enjoy the occasional 5-10 minute round of Lumines on the PSP. Monster Hunter for the PSP is huge in Japan, not so much here, but ymmv.
I guess the biggest problem for the PSP right now is that it doesn't really have that many killer game apps being released at all for it. I think you'll have better luck picking up a $10ish dollar PSN game for it, like PixelJunk Monsters, or that game that you get to literally saw ships in half (don't remember the name), or even pick up a Mini or two (Fortix being a favorite of a friend of mine).