Animal tracking. I picked it up in college and it has been a real source of joy and a true challenge. It's also something you can do almost anywhere: urban, suburban, rural, out in "the wild." A lot of people think of it as looking for paw/hoof marks in the mud, but tracking can actually be quite involved, requiring you to understand the environment and ecology as a whole. For example, tracking birds is outrageously…
Sounds fun! Do you have resources to get started? I do gundog work, it would be fun to do the tracking together with the dog. (I don’t hunt, it would just be to make some walks more interesting)
From there I would just notice other sorts of signs: broken branches, scratchings, scat. Most of my research went into identifying what I was looking at rather than how to track, if that makes sense. Honestly, most animals are pretty easy to track (not to say you'll always catch up to the animal, that can be difficult depending on what it is you're after) a lot of it is just being intentional when you go out and keeping your eyes open.
You can challenge yourself by going out and looking for specific animals. I did this with a falcon that I had heard lived somewhere in a forest near me. I did a fair bit of research into how falcons behave, when they're active, etc. But there are some things I did not find anywhere online.
In particular, when you get into a falcon's territory the woods become silent! No bird songs, no skittering of squirrels, just silence. I didn't realize this at first, but after a few attempts I found that whenever I got into a particular area all the nature sounds went on mute, haha. I didn't know this signaled the falcon, but it made me hone in (I thought maybe I'd find something else).
There are people who are way better trackers than me, I'm sure there are a lot of really valuable books/videos you could watch, but it's really something you can go out and do right now. Again, you need to manage your expectations, the fun is in the tracking, it's a special treat to actually find the animal, though of course that does happen. It can also be a really valuable skill: I once was out tracking deer and came across fairly fresh bear prints; I immediately turned around and headed home, haha.
And yes, I did eventually sight the falcon, her babies gave her away, and she was magnificent!