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.
Other
100th Day of School Activities
These mostly math-related activities are for elementary teachers to celebrate the 100th day of school.