I just did thisyesterday. Walmart had a jar for about 7 bucks, and I just downloaded the picture here and printed it.
I didn’t get around to filling the jar with water, although I did laminate the picture for the future.
I put the jar in the cabinet where the glasses are, so anyone getting a drink got a good scare.
I was there when my wife reached for a coffee cup this morning. Priceless.
Put it in the fridge or a cabinet. Hilarity will ensue. 
If you have acoustic drop ceilings, some mechanical skills, and several evenings available to set it up, you could arrange to dump a carton of ping pong balls on someone.
I guess the old stapler in jello is too passe?
I have 24 hour access, but cameras are EVERYWHERE so I couldn’t get away with it.
Oh, this one sounds like the best!
It’s been a running joke at my company that I’ve asked a couple of times for an office when one comes free, but never got one. (I wouldn’t want one now, but I used to, badly.)
I’ve decided that this year I am going the “Les Nessman” route for April Fool’s. I have some yellow tape, and Monday night after just about everyone is gone, I’m going to build myself an “office”. I think it will go over well. 
Not really a prank, but was just an interesting “experiment.”
At my old job, I created a Word document that simply said, “Do not remove this piece of paper.” I then attached it to the bulletin board in the break room.
Anyone want to guess how long it stayed up?
Post a notice by the printer: Until further notice, to reduce costs please only print one copy at a time.
Make sure to add the date in the corner.
Print QR codes containing the URL to a certain Rick Astley video. Distribute as appropriate. Shorten the URL if necessary, to obscure the target.
One year (at home) my sister switched the sugar & salt. If you have the right setup in your office kitchen, your cow-orkers can have some very interesting morning coffee.
Someone got me good a few years ago. We were doing some software dev and he sent me an .exe to “see what I thought.” I thought I’d be reviewing some functionality for the project we were working on. Instead it set all of my mouse pointers to sex objects. The normal pointer turned into an erect penis, the “spinning wheel” became a boob, etc. I did not realize how many icons Windows actually uses for various pointers until I had to revert them back to normal one by one in the mouse control panel.
What happens if you:
Take screenshot of desktop, rotate it 180° and set as wallpaper
Rotate monitor display 180° - now it all looks normal, right?
Set mouse orientation off by 180° as well (may require special drivers)
I’m thinking the desktop will work normally, but all programs will load upside down?
If you want to do this, you don’t need to use Photoshop. Stand in front of a blank wall, set your camera to panorama mode, turn your head to the side and start the picture. Slowly rotate your head, without moving the camera. It’ll think your head is a panorama.
Posted on the printer nearest my desk (and IT notified in case they get calls). Not sure if I’ll be able to hear, but I hope so…
“Reply to All” email storm…
Causes no physical harm and it is often quite funny.
Best if you use an outside email to start this and that you do so from offsite.
A few years back, I used this online text to voice appto facilitate an office prank. One of the voices sounded a lot like our client. So I called up one of our account managers and pretended to be the client all pissed off.
What made it even funnier is that there is a delay as you type and I had pre made some common phrases. So it sounds very Terminator-ish (…“fuck you…asshole”)
Sorry, but you could only do this by hiding the desktop icons, which would mean only having the fake ones on the desktop, which would be non-functional. And the functional start menu (which I assume you moved to the “top”) would be upside down, as would be the mouse cursor (though the latter would be fixable.)
If this can be done, it would involve hacking the Explorer.exe executable.