I used to love my Jawbone Up for this exact use case. Didn’t want to wear a Fitbit/Apple Watch as I already had nice watches, so just wanted to have a fitness monitor that was subtle.

Tried the Oura ring but nowadays it requires a subscription for pretty much everything. Waiting to see if a nice device comes out that’s integrated into Apple Health, honest have a screen and doesn’t look a bit daft if you’re wearing a mechanical watch.