Just saw an inspired idea at Natural Blogarithms, a blog by Dr. Scott Franklin. He mentioned that his kids' school is having a unique fundraiser. He said:
"Instead of accumulating donations to win for their own grade, they are putting their change in other grades' canisters. The grade with the lowest weight in donated change at the end of the fund-raising wins."
I love this idea. Kids have to think about math as they try to help their class win. Which coins are the best -- that is, which ones have the highest weight to value ratio? You can read Dr. Franklin's analysis here: