Fairly early in the 2.0 beta we had a contesthad a contest based on views; the idea was to publicize the site. That's not the problem we're trying to solve now, but it got me thinking about other contest possibilities. Last winter I saw a contest on another site where they identified badges that are tied to the kinds of growth they wanted, and people who earned those badges during the time of the contest were entered into a raffle for prizes (one entry per badge, and gold/silver counted extra). I was thinking that we could do something like that, with possible badges including the voting ones, Nice/Good Question/Answer (for posts created during the contest period), some of the moderation ones (like Custodian et al - h/t Daniel), and possibly others. This is a partly-baked idea yet.
I'm not sure what a good prize would be. In chat we discussed possibly tzedakah donations in the winners' names. Or, last time we did a gift card to an online Judaica shop.