My first grade team (which is awesome, by the way) always rotates the students through our classrooms so we only have to prepare one great lesson and then teach it to every class. I've done my activity every year and I totally love doing it - it's so dang easy and the kiddos really enjoy it.
Here's what I do: I write the numbers 1 - 100 on those Avery Color-Coding Dot Labels (the primary colors) and then hide them ALL OVER my room. When the students come in I give them a paper with the 100 chart on it, three primary colored crayons, and I tell them they are to go on a number hunt. When they find a dot they color that number square on their paper the same color as the dot (see below pictures it that's unclear). I make sure I also tell them not to remove the dots as they find them so others can find the dots too. They have around 15 minutes in my classroom and they spend the whole time searching. Too fun!
In another classroom they made these cute headbands...
... and art out of the numbers 1 0 0
If you haven't celebrated yet - have a happy 100th day!