You may dress up as whatever you so wish. Common choices were were-creatures, faeries, Dragons, Angels, and cross-race costumes, where vampires would dress as humans and humans as vampires, though those costumes were generally the most inexpensive as only make up to change your skin's colour was required. I have seen many entertaining costumes as well, such as a young gentleman in a blue and white outfit and a mask shaped as a cloud with a bowl of water who would flick you with it as he introduced himself as "Slightly Cloudy with a Chance of Rain".