The children at our school also brought in 100 of something (my son glued 100 peanuts to a piece of cardboard). They also measured out 100 of things (100 rulers, 100pencils, etc.) and then measured how long they were. This was done with Kindergarteners and their 3-4 grade book buddies.
Does the whole school participate in the 100 days event? At our school only the kindergartens do this. It could be fun with the 8th graders too. Maybe they could do a 100 day count down until graduation!
tcw, we do many things for our 100th day however these two things appear to be the most favorite: The children trace their shoe on a piece of construction paper, cut it out, write their name on it and place a piece of tape on the back. Then each class goes out in the hallway (Not at the same time) and counts 100 steps. When each child gets to 100, they can stick their foot on the wall. For one week, our school is covered in "feet". Also, we cut out construction paper in the shape of 100 with the 00 being the eye holes, then the kids decorate their "eyes" and wear them all day. Hope this helps!
When we had our 100th day party this past year nd we had the kids count out 100 pieces of candy, they could wear shirts with 100 pieces of something on it, my son wore a sweat shirt with 100 craft eyes glued on them. They also strung 100 pieces of cereal (like fruit loops) on a string for a necklace. They also had pieces of paper and different colors of paint and made 100 fingerprints on the paper. Hope some of these ideas help you, our kids had a great time.