How about a PHP poker table or blackjack table that is incorporated with the forums?
You would get so many 'credits' per post and be able to play against other visitors to the forum. Maybe invite them into the game if they are online. Of course a chat would be nice, but not really necessary.
The forums already tracks total posts, so the initial credits would be easy. The modified credits would be a little harder unless you change the string to a different one for each person.
Yes, I used to be an old SysOp for a local BBS around 10 years ago.