A few years ago, my husband and I went to a costume party as the Men in Black (ok, A Man and A Woman in Black, whatever
). This one's easy - you just wear a black suit, black shirt, and sunglasses. I also got one of those little keychain lights and put a red bulb in it. People might not get it, though, unless you can recruit a partner.
"The Avengers" movie had recently come out (which sucked - I'm sorry, there's only one Mrs. Peel, and that's Diana Rigg, but that's a rant for another day
). Another couple came to the costume party dressed as Mr. Steed and Mrs. Peel. Suit, bowler hat, and rolled umbrella for him; catsuit for her. But I think people only got that because the movie had just come out.