Mine is mapping. I am a big OpenStreetMap contributor and I have mapped many towns near me that were previously completely unmapped.
I love iNaturalist. I lived out in the woods for several years and would see so many different bugs that I didn’t recognize. So when I discovered it about 2 years ago, I started taking pictures of every bug I saw and uploading them to the app to learn what they were. And then in August last year there was an unusual explosion of mushroom varieties in our yard. That’s the one area where iNaturalist is a little weak as it really struggled to give me good ID’s for a lot of them. But it should only get better with time.
I’ll see if I can find some mushroom photos to share here.
Edit: one of my favorite mushroom pics I got during the mycological explosion: