Videos. Journals. Planners. Calendars. Playlists. Spreadsheets. Letters to open in the future.
Keeping track of the present (in a way that is low stakes and fun) keeps me from dwelling on the past or feeling upset about changing. It will be there for me if and when I want to return to it! And I get to dip into the things I have thought and felt and loved in the past and maybe they become part of my life again or they don't!
I also love to experiment with form in the ways that I record myself. Journals are a classic, but doing something like journaling in a planner with limited space or on a calendar means you have to pick just one memory from each day. Or making playlists / watchlists / reading lists of what's important to you at a time. Embarking upon fun little challenges of discovery (places, songs, books, movies, foods) and keeping track of what you learn in an organized (or disorganized) way! Video journals or little vlogs just for yourself! Scrapbooking!
Fun in the moment to create and fun in the future to rediscover.