I currently don't have much time to put into hobbies, but I did some gardening/landscaping during a break in the rain last weekend. Felt great to get out and move around. Garden finally is put to bed for the winter (or what's left of it).
My hobby right now is making a completely local fitness watch without any screen or distractions. Just logs heart rate, spo2, activity, and sleep using actual Bluetooth standard profiles (HRP, POP, PAMP, etc... Instead of proprietary hacked together custom profiles of most watches). No GPS either because if I want that, I will just bring my phone. Maybe an extension of my job though since I design medical device electronics for work. The only feedback is an LED and good LRA haptics (hopefully). I hope to make it compatible either with whoop replacement straps or standard watch bands.
Bit of a serial hobbyist:
gardening
cooking and making new recipes
weight lifting
running (need to get back into it)
implementing a smart home
growing mushrooms
baking bread
making cheese (stopped because Belgium has almost no non-ultrapasteurized milk anymore)
That watch sounds like an interesting project. Have you heard of the HealthyPi Move? It's a open hardware watch with a bunch of biometrics, so maybe there's something in the design or code you could use.