I'm dressing as a slightly befuddled graduate student. I have the mannerisms down just so: right hand pushes hair back while thinking, eyes staring off into the distance, slight drumming of the right heel. Jeans, t-shirt, and some sort of sweatshirt for warmth. I figure I should occasionally take a break to get coffee or lend someone in hand, just to stay in character.
I also considered dressing as a house plant. But that costume would have required taking greenery away from the plants on my desk. I don't really water them as often as I should, and I think that taking any of the (unwilted) leaves from this poor White Butterfly Nepthytis would just about do it in.