For the bat, paint the can. After it's dry, cut out some fangs and wings using white and black card stock. Hot glue the googly eyes, fangs, and wings. For the wings, make sure to hold it steady while it dries to make sure they get on really good. Every year or so they fall off after being packed up in a box, but they can easily be hot glued again.
And the mummy, you don't kneed to paint the can. First, hot glue a square of white fabric on the top of the can. Then cut lots of little 1/4 in. strips. Glue on the strips going in different directions. This is fun because it doesn't need to be perfect, just make sure you cover the whole can. Then glue on the googly eyes.