When feasible, on hot days, park your vehicle, so it will be in the shade at the expected time of re-entry. Likewise, on cold days park your vehicle, so it will be exposed to sunshine at the expected time of re-entry.
Also, when feasible at the expected time of re-entry into your auto, park it so the sun will not be shining on your seat on hot days, and so that it will be shining on your seat on cold days, unless you like punishment. Ouch!
On hot days, turn the air conditioner, vent, heat, and defroster selector swicth to the vent or heat position, so the car can breathe and not allow as much heat build up in the car caused from sunshine. (On the majority of modern vehicles all vents will be closed when the selector switch is in the off position). Also having the windows rolled up, but not snug tight, can allow additional routes for heat to escape.
On cold days, placing the air conditioner, vent, heat, and defroster selector swicth to the off position, can give the sun an opportunity to build up a little heat in the vehicle.
Having a car with at least a white or a light colored top will typically be more suitable for the warmer climates, and of course a black or dark colored top may be more preferrrable in colder climates. Naturally, dark colored objects absorb more heat from the sun, than do light colored objects, which reflect more sun light.