foam mat roll, exactly like this, from harbor freight (or wherever).
cut it into squares or narrow strips, or micro(for easier packing).
why: save your butt. softer than sitting on rocks, gravel, or concrete.
benefit: cheap, light, effective.
cons: easy to forget (thus leave/lost risk) , often packs awkwardly.
*tip: large pieces double as shading tool eg when walking in the sun.
It’s definitely important to have something for a sleeping pad, although imo the inflatable ones work better. If car camping another useful thing is to bring loose cardboard you have lying around and line the ground with it before setting up the tent, adds an extra level of comfort on top of the sleeping pad.