Naturally, If you're reading this you have no more time for some big scale jokes, but there are numerous things you can do tomorrow that will annoy the heck out of your victim.
The easiest target is always the computer. There is not a possible scenario they wont check their mail or Facebook at least once, so it will most certainly work. Also, the computer requires no more than 20 minutes of slicing and dicing to become absolutely unusable in the funniest possible way.
The Icon Prank
Yes, I'm aware most of you know this one already, but it's always a good option if you want to troll your victim's desktop. However, I'm pretty sure you haven't yet thought of how to push it to the maximum limit.
First off, steal their laptop or wait for the to go AFK. Screenshot they desktop, so you know how it originally looked like.
Decide which icons you want to stay or not, but definitely delete the most important ones like: the browser, their most played game, messenger of choice or software they use regularly.
Now go ahead and swap all of the short cuts with one another, do it in a random order, so that it's not oto obvious where everything is after the first few trial and errors.
Take out your screenshot and start renaming the short cuts and changing the icons, so that they match the original layout and order, but open entirely different applications.
This is easily fixable, but it will take them at least fifteen minutes and will annoy them to death – there is nothing more irritating than ordering your desktop.