Originally I purchased custom fit reflective sunshades for the front windows and since they didn’t make them for the rear I installed window coverings. The game-changer for me was the custom fit blackout shades for the Toyota Prius.
05/03/2021 UPDATE: PriusOverlanding.com now offers the blackout sunshades for purchase!
Not only do the sunshades help keep the car cooler, but they also allow you to be in your car with all interior lights on at night and not be able to see from the outside the lights are on inside.
The blackout shades even come with shades for the tiny side windows.