Soldato
if you have psn plus then you will likely have a host of games.
the most important thing is get yourself some cheap vita cards off ebay. I don't know why they never used SD cards. it's shocking. if you hack your PSP then you can buy an SD card adaptor which works.
so maybe worth doing to save yourself a fortune on VITA cards.
Although not clarified in the OP, he said one stick so presumably a PSP and not a PS Vita. The use of custom memory was a bit crap for both generations but understandable being how readily people are willing to pirate games, especially handheld.
GTA Vice City Stories really enjoyed.
Ridge Racer
Tekken
Wipeout Pure
I recall the Syphon Filter games being excellent when new, not sure how they have aged though.
Loads of PS1 games
Want to get my 1000 out again now too