I like wrapping myself up in a costume; it's fun to be able to escape into that other mode of self, where you aren't you: you're the party version of yourself, the version who's given themselves licence to be just a bit silly.
I've been leaning on a single costume for much of the last decade, one that works for me on October 31st and at the Renn Faire equally well: A Wizard. I have a staff, cloak, large arcane tome, and boots. In some ways it's more of a solid start to a costume, I have all of the ephemera, but not the shirt and pants. I hope to pick those up before next summer.
This year, I'm going to try and pull off a pirate outfit. I've got a blue corduroy jacket with red lining that's been vaguely piratical for years, so I should be able to pull it off with a certain amount of ease.