Scholastic has put together 100th day math ideas from several different teachers. Includes "zero, my hero", probability, sorting, measuring, estimating, counting, weighing, money, and other ideas for celebrating the 100th day.

Early readers can practice their counting skills with this short online storybook based on the hundredth day of school.

Great ideas for "100th Day" activities. Most of these would be a good challenge for upper elementary school students if you want to include them in your 100th Day celebration. Activities include counting, measuring, weighing, sorting,...

Scholastic.com provides a reproducible coordinate grid activity on a 100 grid to help your students celebrate the 100th day of school.