how would you guys feel about contests or challenges?

rough examples of contests:
creative (art, graphics, videos) - popular or staff voted
website (guides, info, solutions) - popular / staff voted or number based

rough examples of challenges:
fitness (30 days, a week, etc) with or without others
dietary (30 days, a week, etc.) with or without others
fashion - building a wardrobe, donating, etc. with or without others
charity stuff
etc

some would have money prizes, some would have rewards and cosmetics, some for pure love of the game.

would any of these concepts be interesting? what elements sound good, which don't?