I'm going as a Brownie this year. I'm not shaving the week ahead of time, to add into the extra funny.
I went as Pretty Pretty Princess last year and as a Toga which turned into a tutu that looked like Jim Carey in Ace Ventura.
The year before three of us went dressed as sperm, in white painters suits. My girlfriend at the time gave us the brilliant idea to carry sharpies, so we had girls sign us. They left awesome sayings like 'cum on my face' and 'fuck me in the ass', you know the sort of things that would make dad proud, and then their names, phone numbers, and this was when facebook just became popular so they left facebooks.
We "swam" around all night like sperm and took pictures with a ton of people. We were the hit of the party and the streets.
...the costumes originally were supposed to be Scooby Doo but a Scooby costume turned out being $70, then we switched to Ghostbusters and when we put the suits on a friend told us we looked like sperm - so thats what we went as. It was cheap and fun.