Somewhat unrelated, but window awnings are an often overlooked solution for your exact problem. They're not quite as quick and dirty as some reflectix or aluminum foil, though.

https://youtu.be/uhbDfi7Ee7k

Yes, covering the window will reduce solar heating by a lot, even if you just use basic blinds or curtains. You can also get stick-on window film for cheap that might work better than blinds. You could also use a low-profile fan like this one to get airflow without exposing much of the window to sunlight.

foil might not be worth it. if the sun is hitting the foil on the window, it's going to heat the foil which will heat up the window. if your window is multipane, it can deform the seal and let the gas inside escape, which obviously greatly reduces the insulation of the window. better to use paper, since that won't get as hot. plus, you can paint the paper with a design to make it look nicer.

of course, the best option is to prevent the sun from reaching the window in the first place, such as with awnings or strategically placed obstructions of other sorts, such as plants or decorations. if you're handy, then shutters aren't terribly hard to make. any amount of sun that's blocked from reaching the window is more heat that you don't have to worry about.

Depends, sometimes hot air gets sucked into the room - but in general it probably will improve your situation a bit. Can you also leave a door open to create a draft? If you are lucky you can pull cooler air into the room. Best try. Covering the sun away is always a good idea.

Look at aluminet. Woven thicker aluminum foil into a reflective sheet that wind can pass thru. You can specify the weave density to choose the % light blocking for your preference of wind and shade trade-off. I’ve got a 70% blocking one and use it a lot for camping in the desert. Available in different sizes and they come with a border that has ring holes for securing.

Can also put a fan in window and the shades used for car windshields around that to block the rest. Even better is to have a 2nd fan elsewhere to pull/push the air thru the building on the other side of your place.

I've just done an experiment with horticultural sheeting on all my (huge) bedroom windows and it did not work. Any type of reflective material would need to go on the outside of the window to work.

If you are able to, give it a try and see what happens.

In our rental houses, we have always used foil on the north-facing windows (I’m in the southern hemisphere) in summer. Leave a window open an inch or so to allow air flow, but also open a window on the other side of the house so you get a draft from the cooler side to the hot side. Sometimes nights can be cooler; if that’s so, open as many windows as possible at night to cool the house, and close them again before sunrise.

Reflecting the sunlight away will reduce the heating you get from the sun. As far as opening a window or not, open if outside is cooler, else close.

If it's pretty windy then it may be worth opening the window if the temperature is similar.

Summary of answers so far:

• Yes.
• No.
• It will break your windows.
• Do $other-thing instead.

I hope OP found clarity...

Can you install a ceiling fan? They work really well at moving air, and one can change the rotation to push warm air down during winter months.

cover the window in thermal reflective film. You can buy these at most hardware stores, make a huge difference and only reflects off infrared heat.

Preventing light from getting in will help. Opening it is really tricky to say. Depends on the ac. Best bet is to try both approaches and see. Opening and closing is easy. Bit the paper or foil WILL help. Especially if it gets a lot of sun.