....Scratch card rewards.
These are so simple to make (although not exactly cheap) and apparently the students absolutely love them!
Just make your cards with various prizes on (lolly pop, sweets, chocolate bar, house points, stationery etc.). Then cover up the prize using a sticker from Scratch Labels
or make your own. The labels should scratch off really easily and the sticker part is clear meaning you can see the prize underneath.
I'm going to trial this with my tutor group first of all and will keep you updated on how it goes.
I also like @SarahPym's idea of laminating them so they can be re-used. Would be a good way of saving money on printing costs.