I recently turned one wall of my house into a Space Invaders game with the help of Blik, a company that sells decals you can stick pretty much anywhere for instant awesomeness. Turn a blank ceiling into a star field, or your bed into a gothic fantasy zone. Here are a few of the patterns Blik has right now, from gorgeous fantasy illustrations to crazy robots, that you can grab and stick over your desk right now (provided there is a wall over your desk). Check out Blik — most designs cost between $40-$80, and I can attest that they look great and don't peel off. They also add new designs all the time.
Bonus for the reference to the William Shatner Twilight Zone episode.