Here are a few easy and original ideas:
A Douche Bag - get a big trash bag, cut holes for legs, arms, and use the opening for your head - duct tape a poster board with the word DOUCHE in large letters to the front & back.
A "One Night Stand" - a cardboard box decorated to look like a night stand - glue a lamp, picture frame, and maybe some condoms to it. cut holes for head, arms, and legs.
The blues brothers - If you have a black suit, a black tie, a black fedora, and some sunglasses, Find a male friend to dress up alike