Coupons I made to use as classroom rewards.
Students can put their name on their coupon to save until they want to use it. When they use it, I put the date and initial. If it's something like wear a hat, they can keep the coupon to show to other teachers when they switch classes. Usually the reward is just for one classroom though, to make things easier.
Sometimes I'll give just a coupon as a reward, but usually my students have to buy the coupons with their classroom currency.