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 not complete the 2nd side of any worksheet or 2nd page of any math book assignment. Of course, you can modify this however you would like.
I had 6th graders and they LOVED the coupons!