I sometimes carry fake lottery scratch-off game cards in my camera bag, so that when I am finished shooting a new model, I give them a ticket as a thank you, and then have them scratch the ticket to see if they have a winner.
Of course they will scratch off three $10,000 prize markers and think that they won $10,000 - it's always funny watching their reactions - from excitement, to disbelief, to big excitement when they think they actually won $10,000, to slapping me when they realize they were pranked - AND it's all on video!!!
Good times!
ADG