my 31/2yo gets to do special activities that the others don't get to do, because he's the only one that doesn't put the things in his mouth-a big incentive for the others (i hope!) He does "tray activities" at the table, and I have him blocked off with a gate so the little ones can't get his stuff. He does things like the activities on this website, and he LOVES it!
(I think I got this link from someone on this board
http://lapbooksbycarisa.homestead.com/TraysforTots.html