I have been concidering the whole tint your windows for a cooler car which will lead to less AC use which gives better MPG....
Then I thought how about a solar panel that can slide into place while parked to recharge a booster battery or power an exaughst fan.... In the door windows it might be possible to include a second thin film solar pannel sheet that could be raised or lowered so you can have a clear side window if you like , or you can pull up the solar sheet while parked to block out the light and produce a tiny bit of solar power too.... the front and back windows are a bit more of a challenge with this approach but it might be possible to rig a solar sheet to come down on a track from the interoir roof of the car to cover most of the back and front windows... I haven't seen any one on line wiht this done but have concidered it myself as a back burner project.
Then I wondered about window replacements that were tinted and had the solar built in to them... so I searched a bit and there are sompanies that make power glass ... glass that is tinted but the tinting doesn't just block light it converts some of it to electricity... one company is at ... www.xsunx.com
The last alternative tinting I thought of would be something that can block almost all the light while parked but can be more clear while driving so the option along this route would be to replace the windows in the car with windows that can be tinted as I see fit when I see fit ... www.spd-systems.com
it is a flash site and the audio is loud so you are warned but they have a automotive glass that can be tinted as you see fit when you see fit.
I know while parked a car cover could work too but I hope to eventually hav something built into the car.
Any other ideas people have that I have missed?