Do you know what Schrodinger's cat looks like? Can you imagine how you would dress up as a Higgs Boson particle? Both are potentially difficult Halloween costumes since one might be dead and the other is hypothetical.
But The Science and Entertainment Exchange's blog makes it easy! It spells out how to create the aforementioned costumes (put yourself in a big box; put on a hard hat and a white "God" beard, respectively), as well as how to be Pavlov's dog, climate change (which is my favorite: dress up like a globe and complain about being hot or cold all night), and evolution. You will impress all your nerdy friends, and completely confuse everyone else!