One word:  Contests
People *like* contests, games of chance, etc.  Look at the participation in guessing when we reach 22,222.  Lots of participation.
So, I suggest contests -monthly contests, weekly etc.  Prizes don't necessarily have to have actual value.  Perhaps winning a status, i.e. a 'title' for the week.  Or, a special earned title.
You could also do game contests.  For example:  come up with easter egg icons.  Have a moderator insert them into random posts and people who respond first to finding those eggs has to post something like "Blue Easter Egg" or whatever color it is.  Moderator keeps an egg count and after so much time the person with the most eggs wins.  Or, a color egg has a different value, etc.  Many variations can be implemented.
Could also do virtual trophies.  So, winners of contests can display their contest trophies in their signature or on their webspace here and link to it in their signature.
Planning can be done in an admin / moderator section that is private to members.  Then, roll out when ready.
Um, ok, you get the idea.  It may sound like a lot but it really can be done.