Well, we started off the day by making 100th day hats, complete with label "I made it 100 days of kindergarten!" The hat had 10 strips of paper, with 10 stamps on each strip, totaling 100. Later in the day I gave them a 1, a 0, and another 0 (100). Then I asked them to make a picture with the numbers. The kids came up with some super imaginative pictures; a bee, a horse, giraffe, skateboard.
We also spent some time gluing 100 things to a 100 grid. It took a long time, but they turned out looking so neat.