Doesn't need to be candy, necessarily. Money's the first thing that comes to mind, but if that's prohibitively costly to maintain while keeping the prizes attractive, you could play with probabilities: build a set of diverse prizes of approximately equal value, say, and instead of money or candy distribute tickets to a raffle where the winner'd be able to choose one prize. That might have some funny consequences in this experiment, though, depending on the quirks of how people think about probabilities.