Prinny: can I really be he Hero? is a fun little platformer. It's a platformr with castlevania jumps though, and a few levels can be frustrating, but overall it's one of my favorite games for the psp
Speaking of castlevania, theres a game for that which has a remake of of rondo of blood, the actual rondo of blood game, and symphony of the night.
Loco Roco 2 is a weird platformer. You mostly just use the shoulder buttons to tilt the world to move the blob like main character. The levels and the game itself are pretty short (plus all the collectibles that come with platformers) so this may be what you're really looking for.
If 3d beatemups are more your thing, god of war: chains of olympus is really short. For me, dissappointingly short, and making still wish the psp had a devil may cry, but eh. whatever.
Have you tried the first psp metal gear, portable ops? I personally liked, though a lot of other MGS fans hated it. The levels in this one are really small, the game is short compared to other mgs games. I liked the whole "pokemon" aspect of it though: you kidnap the enemy forces, then imprison them until they join you, then you can have them spy, develop weapons, be medics or go into the field with snak, hiding in boxes until they're needed. The best part is that none of the cutscenes are much longer than 5 mins, so it's much less ridiculous than other MGS games.
if you get any rpg for the psp, make it Popolocrois. it's rare, but totally worth it. When was the last time an rpg made you cry? I guarantee This game will make you cry at some point, unless you're some kind of heartless monster.
Megaman: maverick hunter X. A few things are different from the original game, but what really makes it worth it is vile mode, where you get to play as one of the villains, Vile. He's a really fun character to use.
If you're going to download from psn, get jumping Flash! or jumping flash!2 from the playstation classics section (theyre both so simmilar you may not want to get both). They're both fun 3d platformers (the first one came out before mario 64, I believe) and have pretty short levels, so this might be the best option. Also, you pilot a rabbit battlemech with ridiculous jumping abilities. It just goes to show there were good 3d platformers before mario 64.
WHew... never thought my psp knowledge would come in handy!