We're all webmasters, we gotta do something related to that as everybody can join than!
What about a SEO contest? I remember Sitedeals did it and the thing was just, everybody bought a new site and tried to get as high as possible with an existing word or sentence.
The smarter people will use something which is simple, the real SEO'rs can try something difficult by using a harder keyword to join in.
That contest turned out really well on Sitedeals, many people joined and everyone was pretty happy
This is just an idea from me, ofcourse if everyone would post 1 idea it would be easier to decide which one the best is...