A little unsung hero of the simple fitness tracker is an altimeter. This enables a device to calculate not just how far you walk, but when you walk up an incline, or up stairs.
An early implementation of this was in the good old Fitbit Ultra way back in 2011. Because it was worn on your belt rather than your wrist, it's probably still more accurate than a lot of today's models.
Trackers like this use an air pressure gauge to tell when you climb up or down. And unlike GPS it uses virtually no power. It's of particular use when you're using light tracking, getting you richer feedback than a pure pedometer while not munching down loads of battery.