A blank hundreds chart works well for a variety of math activities. After filling in the numbers, have the students color in certain squares to show counting by different numbers. For example, you might have the kids color in numbers used to count by two in red or numbers to count by five in blue. A filled, laminated hundreds chart allows for guessing games with the numbers. Choose a number somewhere on the hundreds chart without telling the kids. The students take turns asking questions to guess the number. A sample question a student might ask is, "Does the number end in zero?" If the answer is no, the students draw an "X" through the numbers on the chart that end in zero. This works as a pairs activity or as one for the entire group.
Representing the number 100 with objects helps students understand how much it is. One simple idea is to have each student bring 100 pieces of something to class, such as paper clips or buttons. The objects also work for a measuring activity. Have the kids estimate how long a row of the 100 objects is. After lining up the objects, they can measure to check their answers. For an edible version, have each student bring in 100 pieces of an item that can go in a trail mix. Art projects related to the number 100 add a creative aspect to the celebration. Have each student decorate a number 100 cutout, or use a stamper or stickers to make 100 marks on a paper. Another option is to make necklaces with 100 beads.
Making different combinations that equal 100 reinforces addition concepts. Interlocking blocks work well for this activity. Kids make groups of various numbers from the blocks. For example, a student might make 10 groups of 10 blocks to add up to 100. Money is another format for practicing combinations to get to 100. Have the students make different combinations of coins to equal $1. Another idea is to have kids look through store ads to make a list of items they want to buy totaling $100.
Dice or cards provide a tool for playing games related to the 100th day of school. In small groups, the kids take turns roll dice or flipping over cards. Each player keeps track of his numbers, adding them as he goes. The first person to reach 100 exactly wins. For example, if a player has a total of 94 and draws a 7 card, he isn't the winner because he went over 100. He must keep playing until he gets exactly 100. A relay race is a way to get the kids learning and moving. Each team needs 100 of a certain object, such as 100 balls. The players take turns transporting the balls in relay race-style to be the first team to move all balls.