If you just want to try something basic then you can get the Xiaomi Mi Band for under £15 from most places. I've had mine for 4 months now and I've only charged it twice (they quote 30 day battery life but as I turn off bluetooth and only sync it once or twice a day then I get almost 60 days battery life). Actually I'm just checking now and it says I charged 59 days ago and I still have 41% battery left!

It is very basic and you can't upload the data to any fitness websites easily (you can extract the data manually though) but it tracks sleep, steps, calories and distance pretty well. No GPS tracker though unfortunately.

There's no display but it does have 3 LEDs and you can activate them by lifting up your arm and it'll show you whether you're 1/3, 2/3 or if you've completed your daily step.