Send around the TPS report memo.
And/or the ringers to max, and/or put a bit of tape on the hook button.
At work, only do something that the victim will probably find funny once they find out it’s a prank.
Something like this. It wouldn’t cause any major aggravation and would actually be funny. But I would recommend to actually have brownies somewhere. On the back of the brown “E”'s write something like “look in the bottom drawer of the mail cabinet.”
I once worked for a company that got creative on April Fool’s Day. One year they filled the VP’s office with live chickens. Another year they set his entire office up in the parking lot. The VP was always a good sport, and one of the nicest people I’ve ever met.
Don’t let anyone at work know how much of a fun-hating killjoy you are. You guys are the ones who are likely at the top of the list as targets.
A piece of clear tape on the underside of an optical mouse’s eye.
ETA: Just saw it was already suggested.
You can also take a screenshot of someone’s desktop, move all their icons into a single new folder, and then use the screenshot as their new desktop wallpaper. They’ll be clicking on where their icons / files were, not knowing why they can’t bring anything up.
If somebody leaves his desktop unlocked, see if Ctrl-Alt-Down Arrow turns his display upside down.
I like this. Everyone will get the joke quickly, have a nice laugh, and not waste a lot of time or do something that would be embarrassing.
Set your neighbor’s Windows startup sound to a .WAV of the Blazing Saddles campfire scene. I did that once, and it was such a success that I did it on my daughter’s computer that afternoon. She was really, really ticked!
Am I the only one who thinks that all these ideas about messing with someone’s computer are wildly inappropriate?
Disabling their mouse, hiding their icons with a screenshot, etc—that’s not funny, it’s scary.
Will your co-worker laugh with you? Or just be scared, and have to think quickly to avoid showing his embarrassment/anger?
The brownie idea is good, though. Everybody likes brownies 
Don’t let any of your bosses know what a time-wasting tool you are, or you might be at the top of the list when layoff time comes around.
There are some pranks that are harmless and funny. I’ll bet the ones you pull are neither.
Tell them that internet will be down for three days for cleaning. Seven big webcrawlers will clean the internet during this time, and there can be no users online. Then wait for the hilarity 
I assigned a keyboard macro to a co-worker’s computer, so that every time he hit the letter “q” (which isn’t often!) the screen displayed “April Fool” but then backspaced it to delete it and put the letter “q” where it belonged. It all happened so fast, it was very hard to perceive. He finally had to lean on the q key to see it happening clearly.
I also once snuck in late at night and replaced a co-worker’s desk chair with a toilet. He was a darn good sport, and actually used it as his desk chair that whole day.
I once convinced a new hire who was fresh out of college that the company had a beachfront condo in the Bahamas and that he should go up to the CEO’s office to sign up for his week to use it. (I stopped him as he was going out the door.)
It’s pretty clear to me that none of you guys were ever in the Coast Guard (or Navy).
You got thrown overboard, didn’t you?
Not an office gag but I actually pulled this one off. Saved as many copies as I could of the days newspaper. (7 or 8) Put them in a garbage bag for a year, left for lunch a little earlier and swapped them for the current papers in the “free paper” rack at my local McDonalds. Sat, had lunch and watched the hilarity.
Change the font color in office and outlook to white.
Declan
Or, if you just have to do something, don’t let it be as filmore describes, something that, while duped, the victim suffers for real. You know the phrase: “last straw.”
Suggestion:
One year, I grabbed all of the forks out of the office kitchen a bit before lunch. Everyone came in looking for a fork. It starts slow. Where were all the forks? Other people must be keeping them at their desks. “I think I did see some around our pod.”
Pick the moment and then you can either ha, ha, 'fess up, or have someone unknown suddenly show up with forks or something. You’ll be the office “Super Fork” until Winter holiday celebration.
Ah, the office joker.
That’s you, that is.