Surely they are having a laugh?
For the practical jokers among you, there are some real crackers: talk in meaningless jargon to confuse fellow employees (surely one for marketers); see how loudly you can get people to talk on the phone, by saying you can’t hear them; put Deep Heat muscle rub on toilet seats; use a large machete to point at your presentation slides; and every time someone asks you to do something, say “Do you want fries with that?”.
Or then there’s the old favourite – start rumours. If the Diary doesn’t appear next week, you’ll know some of the pranks went down the wrong way!