Love this!!! one of my favorite things to do at the office, is to buy a birthday card, balloon and gift card.
Give the person the balloon and the card. They open it to discover a receipt for what I spent so they can pay me back. If they are observant, they will notice the $25 gift card charge, yet no gift card was given to them.
99% of these people are like deer in headlights. it's hysterical.