Maybe a competition to see who can write the best ad copy for a product of your choosing? I think that sounds like a pretty novel idea. It will get some good ad copy for those of us who are trying to learn to read over and learn from as well as give us an opportunity to learn a bit more from writing.
Why not a competition for member with the most number of posts for the whole month of November. Of course there should be some guidelines like number of words per post or quality of the post maybe.
How about a quiz open to all members with a minimum number of posts? It could be a quiz on music or other thought provoking topics. The proposed procedure is simple. The quiz master will post a question daily for 5 days for instance. For each question there is a corresponding point. The members with the highest points earned after the quiz duration will be the winner. Prizes at stake could be monetary or non monetary.