This would make a cute writing prompt!
Using Tagxedo type in your favorite 100 words: (Tip: I found a picture of "100" to upload into this site)
ABCya has a cute 100 Snowballs interactive in which students can build a scene with 100 snowballs. Click the image below to take you to the site:
Have your students build something with those 100 snowballs and then take a screenshot of their creation. Upload their picture into a program like Pixie and have them annotate/draw over their creation and add a voice over.
Create a collaborative Padlet wall and have your students write all the ways they can think of to make 100. This can be differentiated depending on the level you teach!
UPDATE! Use the "old lady/man" MSQRD app video filter to record a video. I learned about the MSQRD app in Smartphone Marketing School this summer (July 2016) and thought it would be fun for students to use to record themselves reading a poem/something they wrote or talking about what life would be like if they live to be 100 years old. The app is rated 9+ since some of the filters are slightly violent. If I were to use this app with really young students then I might get a volunteer to pull the students out into the hallway one at a time and have the filter already on the screen ready to record. (I'm reaching out to the app company to see if they can make a child friendly version)
Those are just a few ideas! Do you do anything FUN for the 100th day of school?