Have we suggested treasure/scavenger hunts before? I can't remember. Anyway, this was spurred on by the Battle Arena idea. I like the idea of competing for things against other players, but all that fighting doesn't seem very friendly. So how about a Treasure Hunt.
Players sign up in a central location - perhaps the whole thing is run by a Vendor somewhere. Once a bunch of players sign up to the challenge (say a dozen people), then vendor launches the challenge and announces a list of ten things that you need to bring him. For example:
1 Meat Tetrazzini
1 Green Sno-cone
3 Flummery
14 Groddlene
1 cup of water
1 hog-tied piggy
1 Sparkle powder
6 White gas
1 Wavy Gravy
4 Piggie plops
and so on. So long as it's a pretty diverse set of things, it's unlikely any single player would have all the necessary things to hand, so everyone would need to do some hunting.
It doesn't matter how the players get them - they might already have some in their inventory, they can collect them from the streets, buy them in auction, make them themselves or barter with other players. But the first person to return and present the vendor with the appropriate set of things wins.