Sadly, I fall into this category. I still don't know what I'm going to be for Halloween or what The Precious will be. I'm leaning towards Bellatrix Lestrange for me and a yard gnome for her but am not sure yet.
Anyway, Ecouterre has some great ideas in case you need a costume and can't come up with anything. Best of all, these shouldn't cost you much and you will probably already have most of the components in your house!
7 Last Minute Halloween Costumes Lurking Inside Your Closet
My favourite is the Brawny Man. The only downside: it doesn't really work as a costume for us women.