Geothermal Vents. You could use heated water, steam, and just the hot ground for heating. You could use hot gaseous material expelled from the ground (through a vertical vent) to generate rotational power or even use the hot gas to propel something like a hot air balloon into the atmosphere for short periods of time, or have the balloon anchored or tethered to the earth (rope) with a tubular connection to the 'air' balloon from the vent to maintain its aerial advantage over other 'tribes' or for some other mechanical advantage. Gaseous/liquid material can also be used for other things. Sulfuric in natural (or perhaps not on your world) it could be used to produce an acid based material, and then with mined metals, you could chance upon electricity via the 'electric' or lead acid battery. The acid/battery system could be contained (pottery) and available for excursions from your home. Defense - you could use gaseous/liquid system for defense against other tribes, animals, etc. Manufacturing - the gaseous/liquid materials could be used in materials production. Perhaps your system also produces Nitric acid, allowing/resulting for fertilizer production and greater food growing opportunities near the vents. You would find many useful materials 'crystallizing' out of these vents as they vent into the air, water, caverns. Your civilization(s) near the vent may be more productive/successful than those trying to survive far away from them.
I could go on and on. Lots more to add - let your imagination inspire you.
Georg
