Change of clothes, a full set of layers... ( short sleeve, long sleeve shirts, long socks, underwear, beanie, ball cap) a few protein bars, trail mix, meds, pistol, holster and ammo.water. A fire starter stick, chem light and or flashlight and batteries... Not a light for constant use, one for use in short bursts to see when you need it. Maybe some other sundries.
I ALWAYS have a fleece and good jacket (soft shelled hooded, military type, layer friendly) in the truck... Even in summer.
My go bag is a 3 day pack... Always in my truck, sans pistol... I also have a small bag for the wife and kids... The luxuries of having a truck... Space.