I've got a five-year-old son that loves Calvin and Hobbes and drags his homemade Stuffed Hobbes
For Halloween this year he wanted to be Stupendous Man, which is Calvin's superhero alter-ego in the strip.
I was excited about his choice this year because 1) it was awesome, and 2) it was super easy. The costume consists of the following:
Hobbes Doll - Optional. You can make one with the pattern I've included in my Hobbes Doll instructable linked above.
Red shirt with black stripes
Superhero cowl - Use my child-sized superhero cowl pattern. If you need a bigger one, follow my instructions in step 3 to make a cowl pattern for adults and especially fat-headed children.