I'm probably going as Waldo this year (from where's waldo, the children's book); it's a pretty easy one and easy to spot out.
Other ideas I thrown around before doing that:
Fidel Castro (wearing an adidas jumpsuit)
franzia box wine
milhouse [simpsons]
etch-a-sketch [with dry erase boards on both sides of you]
Kim Jong-Il (pretty popular among the tfp'ers, i guess]
Pac-Man
Ali G/Borat
I remember, last day of high school, a kid dressed up as Link from Zelda for the hell of it. He was damn hilarious [went around pushing garbage cans

]
cya,
will.