We like for our kids to come in that morning to a banner, and butcher paper does the trick! Quick and easy, and the kids think it's the coolest thing ever! Here are a couple versions we have done.
One activity we always like to do is 100 Ways to Make 100! All you need to do is hang up a big piece of butcher paper and let the kids write 100 ways to make 100! You can do this a couple of ways... let them come up throughout the day and write their ways to make 100, or give them each a couple of sticky notes to stick up. I personally like giving them sticky notes, that way they aren't having to wait on each other and it's done a lot quicker :)
And of course, we have to make ourselves when we are 100 and then write about it! I know there are old age "photobooth" apps you can use now, but here are some easy peasy craftivities if you don't have time to print photos of your kiddos! Or you could always do both :) You just need some construction paper and cotton balls to make these cuties!
We do lots more activities all day long, but those were just some free, low prep/low copy ones :) You can read more about our past 100th Days that have more ideas by clicking here!