Read Write and Think is a fabulous resource of lesson plans and ideas available to teachers. I found this idea there about two years ago and had a great time completing it with my firsties.
Each child fills a 20 ounce clear soda bottle with 100 items that are all the same....like 100 Cheeries. They store the bottle in a brown paper bag where no one can peek. Then they complete a clue sheet with three clues to help their classmates guess what's in their bottles.
My kiddos made a movie from their projects. It was made when I was first learning to create movies, so it's not the best. But you can get the idea of what the kids do.
I thought it was a unique way of celebrating the 100th day.
Here's the link to the Read Write and Think lesson plan that has all the downloads as far as the clue sheet and the directions. I think there's also a list of suggested things to put into the bottle.
Here's the download for the letter I'm sending home to parents.