I once saw a high school director who had something similar to what you mention but I think he used gift cards to fast food places.

But he had something added to this that I really liked and his students really liked as well. He put charts on the wall near the front of the band hall with everyone's name on them and he devised a system that used the different colors of belts used in martial arts. All 12 scales full range from memory was the black belt level and he had different color belts listed for each number of scales they mastered. Everyone had some color of belt and the kids really got into trying to get to a black belt with all 12 scales mastered.