I went as a windmill this year (or a helicopter if I was on all fours). I was at a loss for ideas, but I wanted to incorporate some kind of functionality and I've always thought it would be cool to have something that could spin on my back.
To make the costume, I taped pieces of cardboard together to make the propellor blades, cut a hole in the center for the axle, and inserted a water bottle through the hole to give it a free-spinning gimmick (my favorite kind). Then, I attached another piece of cardboard further down the bottle and cut a hole in each corner. Then I threaded green drawstrings through these holes so I could wear it like a backpack. I ended up having to cut the water bottle in half because it wouldn't fit otherwise.
Despite not being as complicated as some other costumes, the free-spinning gimmick was popular with a bunch of people. Unfortunately, being made of cardboard, it was pretty flimsy and got dismantled fairly quickly, and I didn't think to take a picture of it beforehand. I drew it up real quick with GIMP to give y'all a better idea of what it looked like though.