Keep a journal or diary. Add daily entries for 100 days. Have a disposable camera and take two pictures every day, child choice...but must choose carefully: one picture that shows what the child did that was the most fun/interesting, and one that shows what he/she did that was scary/or challenging, or at least, a favorite thing. Use the pictures to enhance it for a scrapbook effect, and then write a caption for each picture, with date. Take a picture of the child at the beginning of project, and at various stages throughout, so he/she can see how they change over this period. Have him/her write their name in their own handwriting at the beginning and the end, to place side by side and compare. Measure their height at the beginning and again at the end: how much grown in 3 months. Teeth lost. Put that onto a visual ruler (for scale) to affix into the scrapbook. Have them save small treasures and items that can be glued in, with captions the child writes telling what that item is included for,what it means to him/her.
Put it all into an album for presentation at the end. Add student drawings created during this period, favorite lunch item wrappers, description or picture of a favorite outfit, best friends made that year, sad things that happened, glad things that happened. Include copies of report cards or progress reports or thank you notes or birthday cards received during this time period. In short, the child is learning how to capture and document a snapshot of his/her life during that time. This is over 3 months' worth of days.
Include events both in and out of school, during the week, and at home on weekends.
Teach the child how to choose fonts, sizes, colors (if you have a computer with color printer and they are mature enough to do so), making cutouts of titles, captions, labels, etc. and then rubber-cementing them to the album when he/she is assembling the final product.
This will become a keepsake to enjoy when he/she is an adult, to share with his own children. The child might even want to keep this up over the years if they wind up captivated by such things, and it will reveal stages of growth and personal development over time. It could be a prelude to learning how historians learn about people from the past, valuing and documenting a life with primary sources. (These things are your student's primary sources!) Maybe this would lead to an eventual History Project when older.