The history of food. It's such a fundamental thing - we literally need it to live - and yet it says so much about us: our societal status and what we want to say about ourselves, our background, our culture, fads and fashions, scientific developments, political policy, education and public health...
Other things that fascinate me: space (especially nebulas), timeslips, mycorrhizal networks (as PPs have said, this includes how trees can communicate, I recommend Suzanne Simard's "Finding the Mother Tree"), clouds and weather (I mean, how does the wind blow?) and lots of lots of history. The latter includes the Mitfords, London, periods of change and transition, the Georgian era, the Underground, and the history of my house. Occasionally I find an old picture hook or a fragment of wallpaper and it fascinates me. I think it's social history that grasps my attention: how did we live and why?
I have ridiculously eclectic interests and often find myself down a rabbit hole. I'm off to read up on Mitochondrial Eve now!