I did one of these before. High time for another! Happy Halloween, Everybody!
More Way-Too-Late Ideas for Halloween Costumes:
Ninja With A Broken Arm! This would be easy. Standard, totally normal all-black Ninja costume, augmented by an arm-in-a-sling! Just a regular, white, medical looking arm-sling. Very low-key, but it creates all this curiosity and even drama! This poor Ninja broke his arm. Is he a clumsy Ninja? Bad at fighting? Shouldn't he just lay low a little until it heals? This is one situation where dressing as a Ninja could make you more conspicuous and vulnerable. See how much intrigue that arm-sling adds? It takes just that nice touch of hapless, to redeem the hackneyed aspect and render the costume fresh anew!
Sexy Male Nurse
Kai Den-Zai'Khan from Sever Death Move! There is no such character or video game, but just come up with something outrageously Japanese-anime looking and tell people - "I'm Kai Den-Zai'Khan from Sever Death Move." Get all uppity-superior if they ask you to go further into it. If a real hard core game geek challenges you, tell 'em it's not available in the States until Christmas 2010, but you've got an in with the developers. If they ask you who the developers are, tell them "it's not pronounceable in your language." Then turn on your heel all haughty. But don't walk away! Just stand there with your back turned. That's Kai Den-Zai'Khan's taunt move.
Aggro Safety Inspector! Walk around with a lab coat and a clipboard chewing people out, criticizing unsafe costumes and trying to force anyone without a mask to don "protective eyewear" (you'll need to bring a bunch of big dorky safety goggles with you as props - depending on where you work or go to school, you may have access to these for free).
Socially Delinquent Fanboy! Your appearance isn't important. Just find anybody dressed as Batman and fawn over them. And when I say "your appearance isn't important" I mean that "your appearance isn't important" is an important part of the costume. You want to look like somebody for whom appearance has not been important for decades worth of fashion, months possibly years worth of general grooming, and days possibly weeks worth of bathing.
"Team Costumes" (you'll need several other people in on the theme for max effect):
A Whole Bunch of James Brown Impersonators!