I dressed up as a ghost from Pac-Man once. I took an old umbrella, some shiny blue fabric from walmart, and some duct-tape for the face. It looked pretty good. I cut out the eye holes on the side, so it would always walk sideways. The only problem was, after a little while, it got heavy.
It definitely looked like the Pac-Man ghost, though. That's the only serious DIY costume I've ever worn, and typically, I don't like to dress up or anything like that.