When out of cell range, you need a watch that lasts more then 24 hours and can radio for help. Pebble Time with SurvivalStrap is here.
A beginner project utilising an Arduino and a BME280 sensor capable of giving you 4 environmental readings.
This may be the lifesaver hack, the flex, with a night lamp and a hub of USB peltiers. Keep one with you, always.
Here is an "outside the box" approach to create a compact knife with a comparatively long blade for use in small survival kits.
