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.