Geraniums. And lots of water! You'll have to deadhead them about 1x/week, too.

Kale and mums can be planted now and will last until the first frost and cabbage until it snows. You can also add faux berries and vines if you like. This combo is very popular where I live in Canada, but it depends on where you live and how long flowers last through the colder weather for your choices.

Totally agree with geraniums. Look at ivy geraniums instead of zonal. Their leaves are more glossy and they bloom year round here in Southern CA. They're wonderful for window boxes and other containers since they grow up, out and over. One of my favorite varieties of ivy geranium are the "Blizzard" but there are others.

Please no fake anything but I do agree geraniums are great in the heat I mix them with sweet potato vine in big planters on my very hot s. facing deck.

They can definitely take full sun. I also have my ivy geranium in dappled light and they're fine there, too. Very versatile and easy. As Sabrina mentioned, occasional deadheading is good. They are truly perfect container plants with their filler-spiller-thriller appearance.

Geraniums are great. But flower boxes are very needy. I love gardening but flower boxes are too needy for me, especially in full sun. They usually need water every single day.

FYI: pansies are a cool season flower. They are good early in the spring and mid-fall. In between those times: no. 70 degrees is about their limit for heat.

Marigolds would be a good late-season candidate too.

Window boxes need Miracle -gro or similar fertilizer often to grow well.