To reduce heat transfer from the sun hitting the windows, cover east and south windows, or the part of them that gets sun the longest. This condo complex forbids aluminum foil. I used white paper, and later pieces of white blackout curtain liner cut to fit.
A shade outside the windows works even better. One house I lived in had a long roof overhang. I hung a bamboo shade from the underside of the eave. I let the shade down to keep the morning sun from even hitting the windows in the first place. After the sun moved on, I raised the shade to let light in. I’ve read of people using the sun shields made for car windows on their house windows.
You can use bubble wrap as a window cover. It’s good insulation and it gives you privacy while still letting light in. You can buy some, but maybe you don’t have to. If you receive it in a package or collect it from a store that’s throwing it away, you can repurpose it to the windows. It’s useful for privacy on bathroom windows or those narrow windows on the sides of front doors.
Apply it by getting the flat side wet and pressing it onto the window. Or you can spray the window with water and then apply the wrap. It’s wonderfully easy to take it down any time you want to. Once you’ve cut it to fit, if you take it down, label each piece as to what window it goes on, so it will be easy to put it back up.
If you live in a hot area and replace your window screens, look in to the UV blocking screens. They cost more, but they block in-coming heat. They’d be most useful on the windows that get the most sun.
Plant trees that lose their leaves in winter on the south and west side of your home. They will provide shade in summer, and let the sun’s warmth through in the winter. Fir trees planted on the north side of the house will serve as a wind break in winter time. Trees that shade the air conditioner unit will save as much as 10% on your cooling bill.