I was recently looking for one of these and luckily I was given a Fitbit Alta (non-HR). I actually wanted a HR one but given the choice of free or £££’s I’ve done without the HR!
The Alta is really just a fancy pedometer, but what it does it does well, though the app annoys me and I keep getting emails about the fact I’ve been awarded a “badge”… It struggles to pick up cycling but does record running etc well, and integrates with MyfitnessPal well. It doesn’t integrate with Apple Health though, if that’s something you’re looking for – you’ll need another app to act as a go-between.
Before I was given the Alta, I’d been looking at the Garmin Vivosmart HR and Vivofit thingys, as they offer HR and are swim-friendly (the Alta isn’t).