I have each person that enters the room or exits sign their name to a blank list by the door. The person who ends up being #100 wins a special surprise...a 100 Grand Chocolate Bar!
Love the idea of "Look who reached 100" We anxiously await the 100th day in First Grade. Our "100" zoo is a great assistant!
We make a one-hundred-day quilt with 100 squares. Each square is sent home complete with their number and instructions and then their number is decorated by their family. They can put that number of items on it, decorate the number itself...the sky is the limit. Then, we put them all together to make a wall-size quilt for display.
My students bring in a collection of 100 items that they have collected and we put it up on our bulletin board for everyone to see. One year, I had a student bring 100 crickets (yes, they were alive!)...
In 3rd grade, 100th day activities are a bit juvenile. However, we still complete a 100th day packet, which has different ways using money to make $100.00, etc. Our bulletin board display gets started about 3 weeks before the 100th day - and we have heart cut outs with 100 acts of kindenss. Kids are very eager to nominate one of their classmates. It is festive and really powerful in that others have to recognize a random act of kindess!