I am about finished with Daxter, which came with the PSP, but I also have SOCOM, Hot Shots Golf and NFS: Most Wanted. I enjoy them all except for SOCOM and Hot Shots is far and away the best game for killing time.
I would like to pick up God of War here soon but I think I will finish Daxter first. When I went out for CES last January, the PSP kept me sane on the airplane in in the airports. I mostly listened to MP3s but played a little bit here and there as well.