Last year I stumbled upon this fabulous website that gives a great last-minute-guide to halloween costumes. Last year I was able to create a costume from what was already in my closet, while still getting a positive response from the masses. This year, by using the same site, I found another great last minute costume that I'm eager to try out! It's called Robin's FYI and even though the site looks a bit tacky, it's got a lot of good info on it. I even used it to download free Halloween Bingo cards for a job-related event. Definitely a useful site.
If you don't have the right stuff already to create something, you could always take a couple hints from this Saturday Night Live 3-minute scene and embrace a costume in any condition.
Good luck costume shopping & happy halloween weekend!