What I absolutely love about this idea, other than its obvious remarkability, is the way it humanizes the previously automatonized front line worker. Instead of making them invisible, it makes them part of the deal. "Tell your Starbucks barrista a really funny joke, get a free biscotti (knock knock jokes not valid in some states.)"
Why not do this with your accounts payable people? Or give the customer service people the ability to give a prize to the nicest person who calls in each day? What’s the worst that could happen–they might use a little judgment, might enjoy the day a bit more, might even start to care.
If you let them.