Some time ago, I made a call for your best tried-and-tested practical jokes. All I can say from the response I got is this: "You're a bunch of sick, sick puppies." It says something about my readership that most of them involve toilets in some way or other.
* Test a pub or bar for its pikey factor by putting 50p in a urinal and see how long it stays there. Then repeat with smaller denominations before switching to foreign currency. Record so far: 2p.
* Put a sachet of brown sauce under the toilet seat for that mystery "just shat youself" explosion. HP (short for Harry Prickers - 100% FACT!) works best, apparantly.
* Pour a liberal quantity of bubble bath or shampoo in the toilet cistern for a terrifying foamy explosion of mirth and woe. But why stop at that? If your town or city has an ounce of civic pride, there's got to be a fountain somewhere. Use this knowledge wisely, padowan learner. And take a camera.
* Sew a shop's security tag inside someone's coat. Every time they walk into Smiths, HMV or Waterstones they set off the alarm. Hours of endless fun.
* Tried and tested by your humble scribe: Put a sign on a pay machine in a public car park saying "Out of Order, please use other machine". The other machine is 200 yards away and also has a sign saying saying "Out of Order, please use other machine." Works best on pension day or outside a popular tourist attraction. A similar gag also works well with train toilets - with any luck you can get people walking all the way to their destination.
* Fake your death, then disappear for a year. Then next April Fool's Day scare your friends by hiding in their bathroom.
* From Zed's comments via Haddocktwat: Go into MS Word or similar program on co-worker's computer, and configure the AutoCorrect option to replace the word "the" with "the Director is a useless fuckface", or some other interesting turn of phrase. They will usually panic and start scanning for viruses.
* Alternatively, if your colleague leaves their computer, take a screen dump of their current work and set it as their wallpaper. Pick the right person and you can make this one last for several days. "Hello? Helpdesk? It's done it again...."