I think this is a great idea! I particularly like combining an anniversary contest with bike to work month. Here are a few ideas I have:
A Stack Exchange bike to work contest. A lot of companies do bike to work month contests to see who can bike the farthest (or most days) each week and during the whole month. If we could implement some sort of miles/trips tracker, we could have a prize for most miles biked and most days biked during bike to work month. The winner could get a little swag package with the Bikes.SE shirt, bottle, and stickers, and perhaps a nicer additional prize like a light. This part of the contest doesn't directly improve content on the site, but I believe it benefits promotion in a couple of ways: it gets people thinking about the site, coworkers and friends may find out about the site from seeing the contest participants getting buff, etc. Also, we like fun!
We could do a participation or tasks aspect to the contest, like what we had on Super User for the Windows 8 Challenge. This could be something like weekly prizes during May for most questions asked/answered or rep gained.
Cleanup and site improvement tasks, similar to some of the contests linked above. One possible downside is that with the new review tasks, there's a lot less cleanup to be done, which lowers the viability of a cleanup-based contest.
Promotion at local events can help spread the word about the contest and the site in general. Many local bicycling advocacy groups have events during Bike to Work Month, and this is a great chance for us to spread the word about the site.
- I'd specifically like to get a group together to volunteer at the May 12th Portland Sunday Parkways. Groups can volunteer together to "adopt" an intersection on the route, and will help riders and neighbors while also promoting their group. If we can get some people interested in this, we could set up a table at our adopted intersection, hand out swag and info about the site, etc. See the meta post for that event.