On your cupcakes, I mean. Not literally. Unless you need to pick something up from the market to finish them. That's annoying when that happens, isn't it? You end up using lag screws in your baking just to avoid another trip to the store!
For Skullcakes:
Junior MInts make great, quirky eye sockets, halved M&Ms stand in for nostrils, and sliced chocolate chips define teeth.
For Owlcakes:
Halved mini-Oreos make great eyes with M&M pupils. M's also make adorable beaks. Pipe extra chocolate frosting into expressive furrows.
For Mummies:
Red hots are great for spooky red eyes (M&Ms provide other colors), and white and chocolate buttercream frosting do the rest!
For Braaaaiiiiinnnnnns:
Mix red and green food coloring into the white butercream frosting. Pipe zig-zag lines through the cut tip of a plastic baggie or piping bag. If you have it, I imagine a flat icing tip would do wonders here.
Make sure to post pictures of your own fiendish creations. Enjoy!