I like the paper plate pumpkins with pre-cut shapes. (And of course they have to tell you the names of the shapes and how many they used!)
Another idea is handprint ghosts: Prep ahead by coloring a ton of those sticky paper reinforcements black while they are still on the sheets. For the messy version, have each child dip one hand in washable white paint and then press it on dark paper. Turn the paper upside down, and you have a ghost! Use the black circle stickers for eyes. For a less messy version, trace the children's hands on white paper and cut them out. Use the stickers the same way.