For my son's 4th birthday the theme was Spongebob so we made crabby patty cookies. It was a big hit. All we did was take two Nilla wafer cookies at the hamburger bun, a mint chocolate cookie as the burger and used red and yellow frosting to represent the ketchup and mustard. We had some pre-made and also let the kids create their own. This could be done for any theme as it's just a fun, cute idea!