I once went to a pirate themed party as a treasure map - got a blue sheet, folded it in half, stuck a cut out shape of green cloth on the front as an island, cut a head hole in the fold and wore it poncho style. Drew lots of little palm trees and mountains and things on it, and a treasure chest, with gold sequin doubloons, hidden under a flap of paper with an X on it.
Alas, the punch that night was strong, and by the end of the evening I was going round asking people if they would like to lift my flap and look at my treasure.
And I really should have checked just where the flap would lie when I put the outfit on.
As for themes, a letter can be a good one - come as something beginning with P or whatever - people can twist it to pretty much anything they like and it might encourage those who don't want to look too daft And all the explaining what you are and how you've wangled it to begin with P (or whatever) might be a good icebreaker...
