A fitness tracker ring. My experience is most trackers suck at tracking workouts and Oura is no exception.
What it is good at is tracking recovery by monitoring you all the time you aren't exercising. For example the first night after my first HIT session my resting heart rate was 20bpm high compared to my good recovered resting heart rate. Then it trended straight line down to normal and as soon as it hit normal I felt great and wanted to go again.
Why not just use how I felt? It is good at early warnings. Sometimes it looks bad but I feel good. I'll be sick in a day or 2. It also helps me spot behaviors that hurt or help my recovery by showing my stress levels in 15 minute windows all day.
I got it to track down an issue I was having and now I'm addicted to the data.