Survival, players are sent to a secluded map (having multiple maps could make reruns more fun) and they survive waves of shinobi mobs, until they either all die or people complete the final wave. Naturally the longer you survive the better your reward.
History related events wouldn't be too bad either, like something where you take part in the massacre of the Uchiha, the onslaught of the Pain paths or face off against the Kage at the summit. It's an alternative from the Bijuu stuff.
BLN had these challenge NPCs that you'd go to in order to play minigames. Can't exactly remember everything from the top of my head but the Sado challenge for example was this Space Invaders-like minigame where you'd just kill as many hollows as you can in order to get the best possible rewards. I'm sure tree climbing and jumping games featured in the first Ultimate Ninja Storm, just to give you ideas.