Depends on what impression you're shooting for....

If you want to do something creepy, dress up in a costume that is normally reserved for girls under the age of eight: fairy princess, ballerina, ladybug, cabbage patch doll, raggedy ann, effeminate Pokemon character, whatever (though, preferably something needing a wig, hat, or tiara of some kind). This costume, while tailored to fit an adult, should be at least two sizes too small for your frame. Dirty up the costume, maybe tear it in a couple of places, splatter some mustard down the front like you don't know how to feed yourself, and do some half-assed makeup (poorly-drawn circles of red paint on the cheeks for an apple-cheeked effect, for example). Mess up your hair, slap your headgear on askew and you are ready for a night of awesome.

If you're going for sexy, get a white lab coat, some horn-rimmed glasses, a test tube, and some assless chaps and go as cowboy Oppenheimer.