Because it's April Fool's Day and Fun at Work Day today, I could suggest a harmless prank like putting "voice activated" stickers or signs on your photocopier or elevators, but instead I'm going to suggest you just share with your team either the best practical joke you have ever pulled off or the best one that was pulled on you.
And because people either LOVE practical jokes or HATE practical jokes, it might be interesting to talk about where people fall on the love/hate spectrum when it comes to all this foolishness!