I like the idea of the active member contrest. I think it should be based on post quality rather than quantity. I think I already voiced my preference for a 'Post of the Week' contest. It would probably be too difficult to rate the average quality of each members posts over any time period.

I also like the essay contest idea. Bicycle Bob ran one of these just before his site closed down, and it was moderately successful. The winner (our own Stumpjumper) was decided by popular vote, which seemed to work well. If you did this on a regular basis, you could have essays written on a particular subject for each comp. Winning essay (and perhaps even the 'also rans') could be stored on the site for permanent access, or even on a sister site. It could end up being a useful resource.