While there were loads of suggestions (red velvet cake to simulate the blood of your enemies is a good one that comes to mind), I was pressed for time on the big day and stuck with one of my all time favorite cakes that never seems to fail and always seems to please: a trusty chocolate cake from a Halloween Martha Stewart Living Magazine from 2006 (or something like that). In between the layers I smeared a fresh made strawberry puree, and to top it off I coated it in vanilla-lime swiss meringue buttercream (of the Martha Stewart variety). Yes, I know the lime flavour is kind of odd when combined with chocolate and strawberry, but I wanted to try something a little different and see if it worked. The thing was eaten so quick I don't even think anyone noticed anyway!
As for the design... a few ounces of melted chocolate in the buttercream gave it a nice brown that was easy to pipe some fun designs with.
I even went so far as to revisit my days of relative Japanese proficiency and write some kanji on the sides.
Not too bad for a cake done on the fly, eh?