Housing and shelter with passive- and active-heating and -cooling elements for more comfort.
Water treatment systems using heat (boiling or pasteurization), UV from the sun, or a biological reactor / sand filter from local materials.
Appropriate sanitation to keep latrines or cesspits in the appropriate soils and far enough from drinking water to prevent transmitting diseases.
Water-supply systems using either pipes from natural materials (bamboo, for instance; or coopered wooden pipes as were used up through WWII) or aqueducts like the Romans did or wooden flumes like miners used. Also, water wells (or in a few desert settings, a qanat or kariz).
Water-powered equipment to grind grain, pound poi or casaba root. Or wind-powered like Dutch windmills.
Build aquaculture farms to raise fish / crawfish / shellfish in protected, isolated waters like the native Hawaiians did.
Through beach-combing, one can find all manner of modern materials – fishing line, boat parts, lots of insulation, steel, copper, aluminum, every kind of plastic. Therefore, copper wire could be scavanged and with a few magnets primitive electrical generators could be made.