That's a tough one - all my PSP games are RPGs... That's the nice thing about my 3DS - at least I have non-RPG games for that, including the digital stuff.
I'm not above bringing the 3DS along. I usually bring a second console along for the ride. The PSP is the first one because of Valkyrie Profile. I have it narrowed down to a few things. Maybe someone can help me pick.
Final Fantasy III (PSP version). It's one of the few games in the series that I never finished aside from watching someone complete an LP of the NES version.
Final Fantasy IV: The After Years. The sequel to my favorite game in the series.
7th Dragon. See above.
Legend of Legacy. It's basically a SaGa game. I love the series, but I have to be in the right mood to fully play and appreciate them.
Mega Man Legacy Collection. I've finished one through three so far.
Valkyrie Profile: Covenant of the Plume. I played it years ago, but I figure it might be fun again alongside the original Valkyrie Profile.
Tokyo Twilight Ghost Hunters. I still haven't touched this game. I have no idea what to expect with it.
Another thought I had would be to purchase the new Odin Sphere. I heard it plays like Muramasa, which was a blast.