Oh, be sure to leave a ransom note. I'm assuming you don't have the time to cut out letters from a magazine, so here is a free ransom note font to download to your computer.
OPTIONS:
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This kidnapping activity can also work great in the office. You can leave a note saying, "Take a lunch today. Or else you don't get your 3 hole puncher back."
Quick activities like these can certainly energize the workplace. I remember working at First American a few years back and my boss would gather colleagues to do something to my office after returning from a meeting. I'd return with everything turned upside down or balloons completely filling the walkway so I couldn't enter the door. I'm sure there are some great ideas like this, helping energize those Mondays...