I keep sheets of this rolled up to put in my bedroom windows in the summer, because the afternoon sun bakes the back half of the house. Not gonna do much to prevent the house from heating up with the air temp, but it makes for good sun shade.

I got one of those blackout shades that is black on the inside and reflective on the outside for my bedroom skylight - it's made a surprisingly big difference.
I went from waking up burning alive because my bed had reached 50-odd degrees, to being only fairly warm at around 25

This would help but it's kinda stuff you gotta do in advance. Also i think just super white paint would help. Gotta up that albedo, dawg

I did it and it works. You can use the rescue blankets from first aid boxes. Be sure to have the sliver part face outside. It makes a real difference, but if the heatwave takes long enough, the walls will heat up and the tinfoil can do nothing against that. It still keeps the sun out, which would make things even worse. Windows wide open at night help. Icepacks from the refrigerator wrapped in a towel around your neck cool without increasing humidity. Definitely avoid anything that increases humidity inside like wet clothing, wet towels etc. Frequent cold showers, but short or humidity will go up and the body will burn even more energy to get warm again if it cools down too much. Turn off any boiler.

I’m sure I’ve seen farmers limewash their greenhouse windows to lower the temp inside. Could be workable