I'd love this on an apple watch. Would be especially nice to combine with an NFC tag. e.g. when you brush your teeth, just hold your watch to the brush which has an NFC tag, then start brushing. That activity is now recorded somewhere.
It's really not a big deal to open an app, browse to the particular activity, and register it. But somehow in practice it is. It's another chore that I never really get around to doing consistently. And if you're not tracking data consistently, it's very quickly quite useless.
I have NFC tags everywhere (toothbrush, dental floss, water bottle, toilet, pills, plants), and I just tap my phone on them to track things. For the rest (e.g., exercise) I use Google Assistant on my phone and smart speakers.
It's really not a big deal to open an app, browse to the particular activity, and register it. But somehow in practice it is. It's another chore that I never really get around to doing consistently. And if you're not tracking data consistently, it's very quickly quite useless.