You can choose a single theme.

Some people relish the prospect of strolling through memories, and are happy to begin a personal memoir with their childhood and write straight through. For others, that prospect can feel so daunting that it stops them from writing at all. This, as I recall, was an issue for my father, who kept stuffing what he called his “Anecdotage” in a drawer.
What if you selected a single time frame to write about, or one set of experiences? I’ve worked with people who decided only to describe their far-flung travels, or reminisce about how they assembled their art collection, or chose to recall a wartime experience, or simply document their early years in a country from which they emigrated.

The New York Times ran a lovely little documentary, last fall, about a couple describing all of their pets over the years:
https://www.nytimes.com/video/opinion/100000004769329/pickle.html

What I love about this story is that we come away with a wonderful sense of who this couple are – how loving and warm and inventive they are – without knowing what they do for a living, or even where they live. By talking about all their furry and feathered critters, they convey the essence of what makes them beloved, I am sure, by all who know them.