Mermaid/Sea Goddess/Loch Ness Monster: The disheveled, haphazard concoction of fabrics, nets, and shells is what makes this costume work. If you're really dedicated, keep your hair wet the entire night.
Pirate/Elf: Adding some scarves and perhaps an eyepatch will help make the pirate thing more obvious, while some flowers or leaves will lend themselves to the elf take.
My personal favorite, Tron: For the more ambitiously crafty, add blue glow sticks to a black shirt and be the envy of everyone's inner nerd at the party.
Schoolgirl/Librarian/Secretary: The bow-tie adds instant cool points to the standard skirt, button-up, and sweater uniform.
The Universe: How awesome would it be to answer someone with, "I'm the Universe" when they ask what you're supposed to be?
(photos collected from Google Images, Weardrobe, and NotCouture)