Capes are really easy to make - just take an old shirt, cut up the sides from the bottom, angle in toward the collar, then cut around the collar and that becomes the neck hole for the cape. I snip the collar in the front and add a snap or velcro for easy release.
Usually I just use a shirt that already has a fun design on it (yes, we have a lot of capes around here!), but this time I drew on a spider web... Because Spider-Man is Max's favorite. :)