Memory Lane
Start going through all your old boxes, closets, drawers, etc., and gathering up all your mementos and keepsakes.
Document a different one whenever the mood strikes you, digitally or physically. Write about memories it evokes, feelings it might still kindle, fictional stories that come to mind, include images, other ephemera.
If the items have value, keep them. If not, throw them out, donate them, or turn them into other pieces of art, home decor, or DIY projects.
You can affix 2D objects into journals or scrapbooks, or turn 3D objects into wall art, mobiles, shadow boxes, etc.
At the end, you get art, memories, and a less cluttered home (if you're into that sort of thing).