Nope its not April fools but why not prank your MAC friends early, catch them off gurard. I saw this article and some of them are just great:

10 OS X Pranks That'll Get You Beat Up--or Fired

check the article out to see how to do these but here is the run down of them

1 - I Know Where You Are... Always

The Prank: How creepy would it be if your computer suddenly talked to you when you walked towards it? Incredibly. Especially if it said something contextually relevant, like "I can smell you from here. Go take a shower." Well, if you steal the target's phone for 20 minutes, you will be one step closer to setting them up for the shock of their lives.

2 - And You Thought it was Cut and Paste

The Prank: Two of the go-to keyboard shortcuts for a Mac user is Command-C and Command-V. We are going to remap
the shortcuts to wreak havoc on the target's workflow.

3 - Oh Noes. My Desktop is Disintegrating!

The Prank: Riddle me this. If all the windows on your computer suddenly collapsed, would you A) call AppleCare, throw your monitor at the wall, C) fall on your knees in a dramatic pose and yell "WHY, WHYYY," or D) glare at your coworker, because he just pranked you. The correct answer is D.

ok this one I love - check out how funny this one is.....

4 - , I Don't Speak Tagalog.

The Prank: Unless your target is proficient in 200 languages, this prank will work. Change the language of both the OS and the keyboard input to make your target think that his coffee break bent the space time continuum and left him on a different continent.

5 - RickRolled

The Prank: No prank list would be complete without some reference to Rick Astley's infamous "Never Gonna Give You Up." OS X makes it simple to cause lasting psychological damage to the target, solely by modifying the list of startup items.

6 - OMG I Hope I Saved My Work

The Prank: While OS X doesn't have a Blue Screen of Death, the Kernel panic screen is equally intimidating. Make your target think their computer has crashed, when really, it's a screensaver.

7 - The colors, THE COLORS!!!

The Prank: Using Universal Access features for evil may seem morally suspect, but can result in some of the best pranks yet. Turn certain features on to befuddle the target.

8 - I'm Clicking Dammit

The Prank: Upon returning to his computer after a coffee break, the target will find his desktop un-clickable. This is almost more fun than the desktop collapsing, because the possibilities of causality for this prank are endless. Broken mouse, slow computer, angry God(s) could all be reasons why the desktop stops working. Only thing is, it's still working. The target just doesn't know that.

9 - NOOOO It's My Worst Nightmare

The Prank: Safari is no longer Safari. Now it's Internet Explorer. Dun dun dun.

10 - AOL Hasn't Been Relevant Since 1998!

The Prank: It was a long time ago, but we can still remember how that mail notification used to make us die a little inside. When AOL dominated the email space, the ubiquitous 'You've got mail' haunted dial up customers everywhere. Now, the target shall relive their worst nightmare.

Mind you alot of these pranks can also be played on a PC. However I do need to say play them at your own risk and once they figure out its you well all I can say is RUN, RUN AS FAST AS YOU CAN!