When I was six, I put my sister in a cardboard box. When my mom came to see what happened, she saw a box with the words "Reverse Vending Machine" scrawled on the side in a kindergartener's excuse for handwriting. Out of the box came the words, "PLEASE INSERT CHIPS." When I put a bag of chips into the mysterious void, out came a hand, holding two quarters.
I am currently in middle school, and all over the cafeteria are these pictures of this cartoon star captioned things like "Milk is Great!" and "Beans are Radical!" (not even kidding on the last one). No student I know would ever think, "Oh, the sentient blue star is doing it, let's go drink some milk!"
I refined this idea into something that could hopefully convince kids to eat healthy (or, at the very least, not eat junk) at schools. You put in food qualified as "junk" and out comes your 50 cents (or another amount of money). I already have a database of foods and a concept video.