I found an interesting back-to-school activity that gives you a writing sample and becomes a cool classroom display. ChocolateOnMyCranium calls it an "About My Fingerprint."
She made copies of her students thumbprints, printing them lightly so that students could write along the lines of their prints. Click the link to see examples from the class, who made impressive-looking pages with their words whirling and curving across the page, following the unique pattern of each students' fingerprint. Assign it at the beginning of the year, and have students write about anything that comes to mind to describe themselves or their life.
You can use it as a way to gauge writing ability and learn a bit about your students.