When I was considering with some friends what I could be for Hallowe'en this year, one of the ideas that emerged in a fit of brainstorming was "Schrödinger's cat".
To dress up as Schrödinger's cat, or more accurately, the Schrödinger cat thought experiment, you just need a cardboard box and a heavy marker. Use the marker to write "Contents: Schrödinger's cat. Warning: Do not observe." If you really want to get fancy, you can stick a cat doll inside the box to satisfy the people who ignore the warning. (Then you can scold them for collapsing the wave function.) Leave it ambiguous as to whether the cat is alive or dead.
As part of the brainstorming, I also blurted out the costume "Schrödinger's catbox". I have no idea what that would be, though. It just sounded surreal.
Ultimately, I decided that a cardboard box was simply too cheesy, so I dusted off my Nobel Peace Prize costume, which I wore this past weekend to a pumpkin-carving party.