Seconding the Garmin watches, been using a Forerunner for years and the battery still lasts over a week with an always-on display. Does about as much as a Pebble and even has a documented SDK with sideloading. The refresh rate is very low but totally fine for a watch, I think newer models have much better displays. The only thing I wish were better is exporting all the tracked health data in a format I can use myself. Using the official tool for that I couldn't even get heart rate data.