Here are 2 activities (with a modification) that you can do on 100th day!
1) 100 Reasons I Love My School! Hopefully you can get as many classes as possible to participate to equal at least 100 reasons! There are two versions attached, one black and one with a simple writing prompt.
2) When I'm 100 Years old - students draw a picture and write about what they will be doing and what they will look like when they are 100 years old.