Doom is excellent for a bit of FPS blaster action... though i find Hacx is rock hard even on the easy don't hurt me settings.
Gp Pang is a reasonably good, free conversion, suitable for 10 min toilet playing sessions, though i suspect the collision detect is a little off sometimes (bit unfair). If you like Pang, the opensnes version is quite playable, really fun and far more involving than the gp32 freebie.
Fall-down is also a great toilet distraction... the attention span required for a game is just long enuough.
other than that, it's usually SMS, Nester, GP Engine, Open Snes, or FGen32 for me all the way.