It's really hard to nail down what you should keep in there, without knowing more -
Do you have kids? Seat back protectors, floor mats (you already have these!), wipes, sunshades (if not built in), collapsible bins for storing 'stuff', a trash bag hook
Other stuff not already mentioned - snow scraper, sun shade, portable tire pump, extra phone charger cable.
To protect it and not yourself? Apart from the above - a good quality car wash kit (sponge/mitt, soap, spray wax, microfiber cloths), leather protector (if you have leather), bug shield, bumper guards if in a city.