Collecting snail shells. Turns out there are hundreds of land and freshwater snail species, many that are easily identifiable by even as an amateur. Some are common but many are in narrow habitats or restricted ranges, making collecting an adventure. The microsnails might be the coolest. They're insanely small but under a hand lens they can have very intricate shells and they are everywhere! It's like pokemon but irl.

Don't know if it's a hobby, but I'm fascinated with exoplanets, specifically exoplanets that might be inhabited with intelligent life (as we know it).

Every year I will spend several hours catching up on the latest exoplanet discoveries/news, and try my hand at crunching numbers to predict the number of intelligent species in the Milky Way right now.

The cool thing about this is that each time I play catch-up the numbers get a bit clearer. When I started about 15 years ago many of the nested filters to whittle down the final count were very fuzzy, to the point of just being very vague guesses (I think I usually go with 0.1% of planets from the prior filter in that situation). But it seemed like each time I review the latest data the next filter down gets clear. When I say filter I'm taking about things like: planet exists in the habitable zone of its parent star; planet is far enough away to not be in a locked orbit; planet (or large moon) is big enough to have a molten core (and thus a magnetosphere), etc, etc).

Of course, none of this means we'll ever necessarily meet another species (space is so absurdly vast). But it's fun to ponder nonetheless.