Beth Skwarecki reviewed the Motiv fitness-tracking ring and liked it a lot. I can't get over how tiny and inconspicuous it is: it syncs wirelessly with a phone app and needs about an hour's charge every two or three days. The limitations are that it only tracks heartrate and movement, deducing sleep, steps and active cardio sessions.
The Motiv ring doesn't nag you. It doesn't over-analyze. It just tells you when you slept, how your resting heart rate is doing, and with a little help it can keep track of how much you exercise.
That's really all the data you can rely on from a fitness tracker, anyway. Tracking heart rate accurately would be nice, but it wouldn't change my motivation to exercise or my understanding of my own fitness.
It also needs a snug fit and costs $200. [Amazon Link]