I think free rice would be so much more amazing if we could have multiplayer games, where you and a friend could compete over rice to win for your totals. Me and my class often go on free rice, and it would just make anyone want to get on and play!
A lot of people are just naturaly competitive, so maybe even competions with random players would work. I'm not saying it should be wildly complicated or anything, but even the simplist thing can make people go nuts with excitment.
If free rice wants us to make them donate rice, this would definitly be one good idea. :)