I am very surprised that this is not on Instructables (well, as far as I know) yet. This by far the oldest trick in the book, and about the simplest way to get FANTASTIC pranking results.
What you need:
-Faucet that is both used by the public, and has a hose attachment.
-Clear tape or rubber band.
Clear tape is far more effective, because you can barely see it, but it may leave a residue on the handle.
PS. I just read that it has to be new and creative, so i guess I should have read the rules three times before making this.
Step 1: Wrap the Rubberband
Hold down the button, and while the button is held down all the way, start to wrap the rubberband around the whole handle. Twisk the rubberband, and wrap again. continue this process untill button can be held down without any other force.
For those of you using tape, just hold down the button and tightly wrap the tape around the handle. make sure it sticks.
Step 2: Aim and Set
Aim your water gun at wherever the target usually stands while using the faucet, or just the center of the sink. Even if you miss your target, the victim will still be scared silly by the oncoming water.
For an extra shock, run the cold water for a few seconds, and then shut it off. This way, even if they turn on warm water, they will still get a chilled blast, depending on how long after you set it they get blasted.
Remember to prank safely and have a happy April Fools Day!