Well, if you're just organizing a small event for fun, there'd be no need. But if you'd like it to become something on a larger scale, then people would need a reason to find participating attractive. Ideally, if you could get enough people interested in competing, the competitions themselves would become a draw for spectators, which in turn would fuel competition, and so on.

The advantage of that would be it would make it an option that may be around for some time, which it sounds as though that may be your intent. Something to do other than just challenging the gyms and such, at least. A small event would satisfy that to a degree, but possibly only temporarily.

In any case though, a trophy or medal of some sort would be worthwhile. Similar perhaps to the gym badges, I suppose.

*yeah, he's definitely giving this way too much thought now*