In my 4th grade class we don't make a very big deal out of our 100th day like the primary grades do, but we did do the cutest writing activity. I had the students write a fictional autobiography of themselves at 109 years old. They wrote about their accomplishments in life and what they are doing now at 109 or 110 years of age. Their pictures were adorable and their expository essays were awesome! Made a cute bulletin board display.