You want a Garmin.

The Garmin product that's closet to what I want is the VivoSmart 5, but it still has a screen. I want to track fitness, but not have the time of day available. It's a pretty specific requirement I know, but I don't want to know the time if I can avoid it.

Might I suggest the Withings watch. I love mine. I also wanted something rather simple without all the bells and whistles of an Apple Watch. It’s a hybrid smart/dumb watch. It looks like a classic watch but tracks HR, sleep, fitness, etc and you can receive notifications on it if you want as well (you can’t respond however). If you don’t have your phone on you, you can sync the data later on, battery lasts a month or more and it’s waterproof.

I got the sapphire glass model so I don’t have to worry about the watch face getting scratched up.

Edit: Got my watch a few years ago so I have an older model . Don’t think they carry my model anymore (Sapphire Steel HR) but their newest models look pretty comparable and probably checks all the same boxes.

Edit 2: my bad, didn’t see you’re requirement about not wanting to know the time of day. The Withings watch looks like a classic analog watch, so might not actually be what you’re looking for after all. Maybe try the Oura ring.

My mistake. I had thought you just didn’t want the time/watch. You want no display at all. Seems like a simple enough request.