Wacky, Wacky Words
Microsoft Office is about as much fun as doing homework in a coal mine, so it's not a program you'll use much -- but since other people in your family use it, it's perfect for pranking. Recent versions have "Styles" to decorate the text with different fonts and sizes. Normally they're as exciting as falling asleep.
TOO normal! We're feeling sleepy just looking at it!
To liven things up, you can turn anything they type into crazy graffiti next time they use the program. What you do is write some really silly text: make it huge, and bright green, and with the silliest font you can find.
Now all you have to do is highlight the text, right-click on that silly "Normal" and choose "Update Normal to Match Selection." And that's an exciting update!
This will change all the text in any document opened on the computer to use the new font! This works really well, because most grown-ups use these programs every day without ever learning how they work, so even the simplest changes can be a complete mystery.
NOTE: Any good prank should be easy to undo - make sure to save the old "Normal" settings somewhere so that you can put things back to normal after having a laugh.