I got a Pinetime, the open-source smartwatch. It's a bit "stupid" compared to an Apple watch, but it costs under 30$ and shows my phone notifications, a stopwatch, a play pause for music, and an alarm clock. Very simple, but I love it. https://www.pine64.org/pinetime/
Does the Pinetime work decently well as a fitness tracker? I don't care about mirroring functionality to my wrist that my phone can already do, but I want to add as many new bio-sensing capabilities as I can find in an open-source device.
It's got a heart rate sensor and step counter, but I haven't played with it much. I know there have been some 3rd party developments around this. I am unaware of other smart FOSS watches, but I have not looked.