Scholastic
Scholastic: Celebrate the 100th Day of School
This resource presents many 100th Day activity ideas. They are listed by the following subject areas: language arts, math, social studies, science and even a just for fun section.
Other
K 3 Learning pages.com: 100th Day of School
This site features several links for kindergarten through third grade resources for the 100th Day of School. These links include activities, songs, poems and more.