One of the things I miss about teaching Kindergarten is Zero the Hero and the 100th day of School. My kindergarteners LOVED Zero the Hero. He came every 10th day and I really think it helped them with understanding numbers. I also miss the 100th day of school. I love the 100 year old portraits that we made. The idea came from Finally in First.
My favorite 100th day activity was the list of "100 things we have already learned in Kindergarten." The kids liked making the list and it was pretty impressive looking hanging in our hall (I copied it at Kinkos onto a very long poster).