For me, I start at the beginning of the year with a morning math routine and one component of that is a day count-up to 100. Students are asked to represent the day number in digits, in ten frames, as a tally and using place value. With daily practice it is easy to track improvement of student knowledge as the term progressed and they worked more and more independently.
I have never managed to go "all-out" for the 100th Day and make it an all-day celebration - one year I tried but other school demands took away time from the celebration. One activity though that I do love is to get the students to imagine what they might look like when they are 100 years of age. Once the imaginations are running they will then create a self-portrait using collage techniques. Here are some examples from my grade 1/2 class:
I made googly eyes available to them as well as cardboard, wrapping paper, tissue paper, wool, and cellophane. I was very pleased with the results!
Once they have finished they each labeled their work and then it was off to do some writing. I like to do a comparative description where they first describe how they look now and then describe how they will look at age 100. It gets them thinking about the differences and most have always written beyond just what they make physically look like but changes in the way they walk, talk and interests.
If you are an Australian teacher and haven't yet given this a go I strongly recommend at least giving the art a go. It is a good way to use up small bits of supplies and looks fantastic. So far I have observed that the 100th day usually occurs towards the beginning of term 3.