Check out these fun writing ideas from Scholastic:

1. Poetry Puzzler:  You each write five words at the top of a piece of paper.  Then, trade papers and write a poem using each other’s words.
2. Memory Lane: Have your child start a story with this sentence, “I remember the first time I _________.  I was…”
3. If I were…: Write from the point of view of a favorite character or celebrity.
4. Pass it along: Play with a group of people.  One person writes a sentence then passes it to the next person with the paper folded only to reveal the last sentence written.  Keep going until you have a silly story to read.
5. Day in the life:  Write from the perspective of an everyday item, like a sneaker.
6. Make a book:  Collect some favorite writing pieces and make a book by adding a title page, table of contents and about the author page.

Found at http://www.scholastic.com/parents/resources/article/writing-activities/6-ways-to-encourage-young-writers.