Here are some last minute costume ideas:
First....real easy...just take some fabric and drape it around your naked body in a toga-like fashion.
This may take work but the result is fantastic. Just wrap leather or pleather or vinyl around your waist, secure it with a wide belt, grab that decorative sword and voila! Sexy gladiator!
An old shirt strategically ripped makes a great last minute costume.
Finally something to do with that old piece of burlap and rope lying around! Grab a stick and you're all set to woo the party with your daring and sexy costume.