I use these reusable coupons for motivation in the classroom. You could sell them in a "class store" or you can give them away in the class for outstanding effort, participation or any other reason you can think of that motivates your students.
This coupon entitles my students to go to chew bubblegum in my room. This is a HUGE privilege because it is not allowed. Of course, there are rules with this privilege! lol They can only chew it in the room, they must spit it out when they leave.
I had a bubble gum machine, complete with pennies available, or you can have the kid bring his own.