Love this writing prompt -- fix the worst book or movie that you know.
Creative Writing Prompts for Kids and Teenagers: Resources for Elementary, Middle and High School Teachers
Write a story about this picture. Or use as a cause & effect prompt-- half the class writes a paragraph about about what caused this and the other half writes a paragraph or two about what the effects of this picture are. Then randomly match one of each together and read out loud. Kids love it!
1000 quick writing ideas For when you have no ideas at all....