Find a softy floppy frog. Put him on top of the water cooler. People will get used to him being there, looking all normal. Then after a week or two start to put him into funny positions. The folks from the other team will keep putting him back to normal. Though this joke requires a frog, a water cooler and some people who think upside down is not natural for a frog.
You require a a shiny coin (the bigger the denomination the more fun), superglue, a non-carpeted, highly trafficked floor, as much time as you can spare.
Glue the coin to the floor (very securely), kick back, and watch just how many people will notice it and try to pick it up. They will kick it, tap it, try to yank it off the ground, pick at it, and then when the light goes off (for some of them) the look on their faces is the best part.
As an added bonus, this game can last for years if your cleaning staff are anything like ours.
This one works best when you are the adult, may traumatise your child's friend for life, and totally alienate your child.
When your child brings home a friend from school and they introduce you to said friend e.g.. "Hi, Dad, this is John." You are required to say, "Oh, this is John. He doesn't look gay." Your kid will hate you for life !
This can also work in the office, but watch out for discrimination laws.
This one works best when you are a male. When you are in a crowded lift (elevator) with a female friend, turn to her, and in a hushed but urgent tone say, "Well, if it's not MY baby, then whose is it?"
It makes for a very awkward ride for the others, while your friend turns red and tries to stifle her laughter.
Disclaimer: To get the best result, this trick can really only be played successfully once.
This one is so simple it cracks me up.
Pull the keyboard plug out of the PC. Not all the way. Just enough so it looks like it's plugged in but isn't. Sit back, wait and watch for the calls to Tech. Support.
Most desks have phones, and other PC cables around them. What you need to do is get all of the long cables and wrap them once around the leg of the chair. When the person comes into work the next morning and pulls their chair out in preparation for sitting down, their mouse, keyboard, phone, etc, will play follow the chair.
Disclaimer: Avoid doing this with expensive equipment.
This one is really tricky, but if you do it just right, it's work a thousand laughs, and probably a smack upside your head.
Find a companion you think will handle a joke.
Grab one of their desk drawers.
Turn the drawer upside down and get all of the stuff back into the drawer.
Shove and stuff as much as is required.
When they next pull open the drawer, it will dump the entire contents of the drawer on the floor.