Here's an idea for a a quick costume you can make using my Bear Hat no 2, some imagination and a little bit of help from Youtube.
All you need is a bear hat, some white and black(mix the two to make the grey) facepaints, a steady hand and this Werewolf face-painting tutorial from Beatrice R on You tube. It comes complete with scary music and it's fabulous. I did a simpler, toned-down variation - perhaps a bit more child friendly ie. quicker - they just can't sit for too long, and managed to create my very own little monster.
Doesn't have to be a werewolf, can be a Witches Cat or any other scary creature.