I was at the grocery store on Saturday morning, and some of the employees were dressed up in costume. My cashier, a guy, had on a black suit, black tie, black shoes, white dress shirt, sunglasses, and a small flag lapel pin - Secret Service guy!
Is that not the best easy Halloween costume ever?
Here's another easy one: Oversized flannel shirt, untucked and unbuttoned, worn over an undershirt, along with ripped/dirty jeans and unlaced sneakers = Kurt Cobain! If you have a guitar, take it along. If you wear glasses, tape black "x"s over the lenses and go as Dead Kurt Cobain.