Here's an idea for a raffle or game prize that will keep kids invested week to week, shared by David Page, Area Director in Greensboro, NC.
Start with something small, like a foil fast food wrapper. The winner takes it home and adds to it as much as he/she wants. The ball is brought back to club and then goes home with the next winner, and so forth. By the end of the semester your club will end up with a huge foil ball. You can celebrate the progress at the last club by cutting it open. It is pretty funny to see all of the layers inside.
Another idea with a similar premise is to give an actual door as a "door prize" each week. The winner decorates the door and brings it back the next week. It is a lot fun to see the final effect.