This activity ties in perfectly with the first lesson, Imagining Stories from Ordinary Moments, in Lucy Calkins' and M. Colleen Cruz's Grade 4 The Arc of Story. As a teacher, when you can hook the lesson to a real life author and a book, the kids are much more likely to connect with the lesson's objective.
Activity Name: Writing With Your Imagination
Activity Description: Think of something you experienced this week at home or at school. Now imagine the same experience only put it into a story instead of real life. Imagine you are a butterfly or a lion or a superhero. See yourself in a watercolor forest or on an island made of ice cream. If you can imagine it, you can write it. Draw a picture to go with your idea. Don’t forget to have fun!
Teacher's Guide Link: http://www.deborahlytton.com/docs/RUBY-STARR-Discussion-Questions.pdf
Whether it be for one specific student, a small group of kids, or a whole class, finding a "just right" book makes the teacher's and the students' day.