It's really not that difficult to look like you're busy. But even if you're not working, you can look like you're working without much work. All you need is this Web service - nothing to install!
You can mash away at your keyboard - no dexterity is required! As you slide your fingers across the keys, something coherent splashes across your screen. Go ahead - give it a shot.
Bonus points if you record yourself pranking someone into believing what you're doing is real - then uploading that video as a response to this one.