Maybe get a cheapy Xiaomi Mifit.
Syncs to your phone for £15 or less.
I got one to see if I could get on with wearing a monitor all the time and I bought a second when I killed the first diving (It didn't like 30M pressure inside a drysuit cuff, it seems).
Had the two for about 2 years now and wear it all day every day (except in water - The newer ones are supposed to be waterproof, but I'm not convinced).
Heart Rate monitoring is OK, although there's the odd exceptional reading.
They don't seem good at tracking non pedestrian activity (like cycling), but I don't do much of that and for the amount of real value I get from such a device, that sort of money strikes me as enough.
Generally it monitors my sleep and heart rate fairly consistently and, as far as I can tell, accurately.
If you want to track your heart rate while running, I think you'll need a proper chest mounted monitor, otherwise (for me, at least) the Mifit is perfect.
Disposably cheap, much less obvious than one with a screen and accurate enough.
M