Our car has a so-called
Solar Roof, a tinted glass that replces 2/3 of the roof:
Inside there are 3 panels that slide in the ceiling to block off the light.
This helps while driving, and makes it warmer in the winter. But on the top side those panels are black, so they get pretty hot in the summer. So in the spirit of Aerohead's
Solar Sombrero, I glued aluminum foil to a piece of cardboard
and put it put under the solar roof.
This should reflect more heat and make it cooler inside. The panels can still slide closed. I'd like to glue alu foil directly to the panels, but getting to them isn't easy, plus I read that they are very fragile and outrageously expensive (US$400
per panel 
), so I'm having second thoughts. The foil-on-cardboard is a cheap short-term solution until I get the courage to carefully fiddle with them.