A Bad Case of Stripes (this link goes to an online reading of the book by Sean Astin) is a fun, colorful lesson in being yourself and resisting peer pressure.
Camilla loves lima beans, but refuses to eat them because she wants to fit in and all of her friends hate lima beans. She wakes up on the first day of school and her entire body is covered in stripes. The doctor can't figure it out, so off to school she goes. Naturally, the other kids ridicule her and call her names. Throughout the whole miserable day, her body art changes whenever someone makes a demand - polka dots, checkerboard, stars and stripes.
The situation grows worse and none of the medical experts hovering over her can diagnose the problem. Reporters wait outside her home, hoping to catch a glimpse of the girl who is now covered with roots, berries, crystals, feathers, and a long, furry tail, all leftovers of other people's suggestions.
Finally, a friendly old lady knocks on the front door and solves the problem. Camilla decides that no matter what other people think, she's going to eat as many lima beans as she wants, whenever she wants.
The lesson might be lost on younger readers, but older readers should be able to infer quite easily that not being true to yourself results in misery.