I used this frame for the 1st day of school, so all I had to do was change the star to say "100th" and it was ready to go! I print a picture of each child holding this frame on the 1st day, 100th day, and last day of Kindergarten so they have a keepsake of how they grew throughout the year.
As a grade level, we did a 100th day duck hunt! We bought these plastic ducks from Oriental trading and made cards 1-100 to sit next to each duck. The six K teachers divided the ducks and hid them around our classroom during the student's lunch time. When they came back from lunch, we divided the kids into groups and had them hunt through each room for the ducks. As they found a duck, they colored that number on their 100 chart. The students LOVED this and yes, it was a bit chaotic in our rooms and in the K hallway, but it was worth it!!
Since your students are probably going to be a bit crazy during the 100th day of school, why not let them get rid of some of that energy with this 100th Day Fluency & Fitness! This slideshow has students work on naming and recognizing numbers 1-100 and doing exercises every so often.