Rather than some aftermarket dohicky, I have in mind powering the car's ventilation fan to draw in outside air and get rid of the very hot interior air. I'm talking about using a real fan powered by solar cells, not some toy. This could and should be built into the car, in part because if done properly, you wouldn't have to worry about rolling up your windows should it start to rain. The more intense the sunlight, the more electricity the solar cells could produce, increasing the air flow through the car.

On a hot sunny day, it isn't uncommon for a car's interior to push well over 100 degrees F (120 isn't uncommon, either). If you could power a blower with solar energy that could keep the car's interior temperature closer to the outside air temperature, then you wouldn't have to run the air conditioner so much. I'd wager the gas savings from not having to run the air conditioner would greatly exceed anything you'd save by having solar cells recharge a hybrid's batteries. If I'm not mistaken, a hybrid's air conditioning system is powered by the gasoline engine, meaning it can't shut down the engine when you're stopped at a traffic light.