I echo Mad for Reading's excellent suggestion to dress up trick-or-treaters as characters from their favorite books. Kids just learning to read have a greater selection to choose from than you might think.
Of course, there's always everybody's favorite, The Cat in the Hat. But what about dressing up two BFFs as Thing One and Thing Two? This works even better with twins.
Encourage boys to take on the identity of Captain Underpants, a truly one-of-a-kind superhero from the series by Dav Pilkey. Girls who like a bit of glam might appreciate the chance to become Fancy Nancy from the popular series by Jane O'Connor. And don't forget the one-handed horse picker-upper Pippi Longstocking.
Clifford the Big Red Dog makes a great costume for pint-sized dog lovers. If you're artistically inclined, face make-up can really make a costume pop. Use red face paint and a red sweatsuit to transform any basic cat costume costume into that dastardly feline Rotten Ralph.
With so many eye-popping costumes to choose from, trick-or-treaters will be bringing home the loot. Just make sure they share!