One nice thing about the 100th day of school is that it means the school year is more than half over. Use the 100th day as an occasion to begin the countdown to summer vacation. Calculate how many school days are left in the year, and don't forget to account for holidays. Have the class make a giant chart with the number of days left all the way down to one. Use construction paper, markers and paint. At the end of each day, designate one student to officially cross off the highest number remaining on the chart.
One way to help children celebrate their accomplishment is to give them some sense of how big 100 is. Have students make their a giant collage that contains exactly 100 items. Bring in a stack of old magazines. Ask parents for magazine donations if you don't have enough. Assign items for each student to find, and let students browse through the magazines and cut out items they find compelling. You can specify that the items must relate to their experience at school. Have students glue the items on and make a giant collage together.
Students should know that making it through 100 days of school is no small accomplishment. Give students acknowledgment of their great feat with a certificate of recognition. Make up certificates that read "This is to certify that (student's name) has survived 100 days of school." Let students decorate them with crayons, colored pencils, glue and sparkles, and let them take them home, or put the certificates on display around the classroom.
The 100th day of school can also be an occasion for a project that exercises the imaginations of your young students. Ask your students to imagine what school will be like in 100 years. They can think about what kinds of lessons will be taught, how students will get to school and what technology will look like. For older kids, have them write a story of a classroom in 100 years. Have younger kids draw a picture of the classroom of the future.