Low-effort creative work collaborations could be fun.
A 1-page fanfic, a 10-minute pencil sketch. These are the kinds of things that adults can fit into their busy lives.
I'm not very interested, personally, in the idea of a contest. Contests have winners and losers. If someone thinks they're going to lose no matter what, because they don't know how to write or how to draw, then they're going to be less motivated to participate. If we did, for example, a 1-page fanfic collaboration. I'd be more interested in seeing a pdf with all the contributions, than I would in seeing a single entry raised above the others for no real reason.
That's just my two cents, of course.