While my standard fall-back costume is Poppins or a gypsy/pirate wench, Ernest's favorite (since he's had long hair) is the hippie. It's acceptable for school, and easy to march in when the band plays in the school parade.
But this year he wanted to try something different in the evening. So he dressed as a Monty Python flasher, with a BOO sign underneath his trench coat. However, he came home halfway through the evening saying, "People just don't get it," and went back to the tie-dye.