Here are some charming ideas taken from professional photographers, that can be practiced at home for the whole family to enjoy:
A. How we've changed: A tradition worth starting: take a family photo of everyone at the same location and with everyone keeping their relative place. Here is a famous example for inspiration.
B. How we've grown: Choose a childhood photograph (for the kids pick one as an infant or toddler) and recapture that old moment, years later, with the same posture, similar clothes and a like location. Here are several examples to inspire you!
C. How alike we look: Take a closeup photo of each family member's face, keeping the face at the photo's center and parallel to the sides. Use any simple software or application to "cut" each photo at the center lengthwise, and paste a corresponding half of someone else's photo. This example shows a professional, meticulous work, but you may well suffice for a simple result that will nonetheless show the family resemblance. Take all photos at the same time and same place to have the same lighting conditions, which would make the "pasting" much easier.
Aside from that, while you're having a family celebration, why not start a family tree project? - you can choose a free software or platform (there is an available variety), and start collecting information including photos, recordings, videos, "Google Street View" screenshots and anything else you can think of... :-)