Remove core, and slice apples. Consider using an apple slicer.
Dip apples in lemon juice or sprite to keep from browning
Take two slices and spread peanut butter on one side, using the peanut butter to make them stick together. You can use toothpicks to help if peanut butter won't hold.
Stick candy corn, mini marshmallows, or almond slivers into peanut butter to form monster teeth.