Here are some writing ideas, for a 2nd-4th grader, depending on their abilities.
If you could be one age forever, what would it be, and why?
Aliens have landed on the Earth, draw a picture story.
You can only have one food, times one whole week, which would you pick, and why?
Imagine you are a dolphin, write about your day.
If you could zap yourself into a TV show, what would it be, and why?
You will be meeting Batman, make a list of questions to ask him.
Robin Hood wants you to join his team. What special talent would you bring to his team.
You won the lottery. Write what you will do now that you are rich.
You're are Pirate Camp. Write a letter home telling about your time.
If you could be invisible, where would you go, and what would you do?