Just grab the sheet off your bed, its the poormans poormans costume.
Grab couple safety pins and make it into a toga. Which basically allows you to be any Greek/Roman god/philosopher/ruler. Cut a couple eyeholes and viola you've got the classic ghost. Add your pillow case into the equation to cover your head, instant KKK member.
If you want a costume thats a litle more original get a peice of cardboard cut it into a square (but do a very shitty job cutting it) write Idaho on it then wear it around your neck with string. Ralph Wiggum as Idaho, always a crowd pleaser (for the Simpsons fans).