Last year I circulated an e-mail that the IRS, combatting abuse of non-taxable perks such as dinners, junkets, etc.; has instituted new rules requiring everyone to log every cup of coffee and cookies they take from the break room, which will be assigned fair value and added to their paycheck. Several co-workers actually believed it and launched into delightful anti-government rants.
I need something similar this year. Any suggestions?
My ideas would all get you in hot water with HR if you were lucky, the local DA if you were not. 
Change all (or some) keyboards to another language. Easy to fix, doesn’t damage anything
It has come to the attention of Telecom Services that the phone lines have not been cleaned according to the standard maintenance schedule. Sometime during the day, they are going to be blowing the lines clean. Please empty out your wastebaskets and upend them over your phone in order to prevent the detritus from creating an airborne hazard. If your phone rings, under absolutely no circumstances should you answer it.
How about doing nothing. I personally hate this kind of stuff at work. Work is filled with so many arbitrary requests, demands, and directives that you truly cannot know what is a joke and what is a serious request.
Just do nothing. Seriously. No one will feel slighted that they were not a victim of an April Fool’s Joke.
You can look around online for “Voice Activated Copier Prank” to find a number of signs you can post near your office copy machines. It tells users that all copiers and scanners are now voice activated and provides some instructions on how to use them. It also tells them that while in “learning mode” they may need to repeat the command louder. We got a few good laughs about that in my office a few years ago.
If you have an admin person (“secretary”) who the office can’t function without have them send an email saying that they have to leave for a while.
I did this to my co-workers a few years ago on April 1. I told them I was pregnant (super unlikely, if you know me) and it was high-risk so I had to take sick leave starting now and they nearly had heart attacks.
Wrap everything in your own cubicle in tinfoil, not letting anyone seeing you do so the night before.
Then walk into work on 4/1 and throw an absolute hissy fit over it, accusing everyone in sight.
Maybe something about mandatory cover sheets?
I forgot to mention. After the storm of accusations, anger and spittle are over, and everyone is freaked out by your behavior… that’s when you feign a confused look of discomfort and suspiciously reach into your pants and rip out your tinfoil underwear.
Gag’s over!
Leave anonymous Valentines on everybody’s desk.
People still fax?
Or swap out two keys on someone’s keyboard (minor ones, so they don’t notice just by looking).
Or wrap the receiver for someone’s wireless mouse in aluminum foil. Leave enough exposed that it still works, but just enough to be annoying.
Or shrink-wrap everything in his office.
Replace the filling of a few Oreo cookies with toothpaste and place them at random throughout the package. Leave the package out for sharing.
Take the powdered cheese packets from couple boxes of mac n cheese and make “orange juice” with it.
Make a “spill” by drying white Elmer’s glue on wax paper. Peel off and place on someone’s desk (or laptop, or desk calendar, etc.).
This only works with one person and can be complicated to get accomplished, but the genuine confusion of the gagee is priceless.
Take everything the person has open and visible in their office (items on shelves, on desk, etc.) and reverse their order. Everything is still oriented to the things next to it in exactly the same way, but when the person reaches for the item in its accustomed place, it isn’t there. The fact that everything is still next to everything it was next to before gives them few visual cues as to what may have occurred. It will just feel wrong to them.
We pulled this on someone who had been out of the office for several days and it took him 2 full days to figure out the prank.
Cut out a bunch of letter "E"s using brown paper and tell people that there are brownies in the break room.
Put scotch tape over the sensor on the bottom of their computer mouse. They won’t be able to figure out why it doesn’t work…hahaha!
Or, there’s this.
Pretty sure I’ve posted this before, but I had a fairly successful one 2 years ago. The email went like this (based on an existing factoid that actually used to be (marginally) true on CRT monitors, not so much with LCDs):
"As you know, we are always looking to make small efficiency savings as in the long term, this can save the company a lot of money, which benefits everyone. One thing that recently came to light was the amount of energy a monitor uses to display a white screen is more than a dark screen. Given the amount of computer use we have, we can cut down on our electricity bill by “reversing” our displays, i.e. showing white text on a black background instead of the standard black text on a white background. It sounds strange at first but I have tried it and it is just as easy to read, and could save us as much as £20,000 a year if everyone adopts it.
I suggest you try it in Outlook to start with: go to “File>Options>Advanced” and then scroll down to the “Custom Forms” button – click on this and there should be a checkbox for “Reverse colours”. If you’re having trouble finding it, please click on this link for screenshots of the process.
Please reply to confirm when you have done this, thanks."
Quite a few people struggled with it for a bit, or asked me about it, before clicking on the link.
Ask around if anyone’s seen your headphones. Search your belongings and your desk. Look everywhere.
When you can’t find them, let out a sigh, and say, “oh well.”
Then play a pre-recorded audio file of yourself saying things like, “Remeber you’re only human, Slithy.” “Today you’ll be a success.” “Slithy, you are not a failure.” “I trust in who I am. I am Slithy Tove.” “Don’t look for happiness, let happiness find you, Slithy.” “Slithy, let your eyes smile and your actions do your speaking, not your words.” “If you dig deep into your heart, you may just find your true self there.” “Remember to get toilet paper on your way home.”
Then ask around if it’s too loud.
Get in a bit early, go around the office and turn the sound on everybody’s phone down to zero.