You'll probably want to pay attention to plants which can tolerate some cold weather or even light frost.

Verbena bonariensis is beautiful in my yard in September and they survive some colder temps surprisingly well. Calendula is another, and they are nicer in cool weather than they are in hot - I was really impressed with their late season display. Any of the Rudbeckia's would also be a good pick. Later sowings of sunflowers may also work, but in my experience they don't particularly care for the cold.

Here's a photo of a mass planting of verbena bonariensis in my yard, photo taken September 29.
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I was eyeing up those verbenas in the seed catalog, are they easy to start? Beautiful photo BTW. Thanks for the suggestions!

I have best luck with germinating verbena by trying to mimic warm/cool spring weather. I start them indoors, 3 or 4 seeds per cell in 6-cells. I have a grow light setup in the basement which is quite cool (about 50F). I alternate putting the flats on the concrete basement floor, and then on top of the light fixture for bottom warmth. I alternate every day until I see sprouts, then leave them underneath the lights or move them to my little greenhouse. Verbena bonariensis that seeds out in the yard never really amounts to much before heavy frost, I find starting them early works best.

How about annual asters, also called Chinese asters or Callistephus? Those, along with some late sunflowers, were always my September standbys.

If you won't have frost before the end of September, sunflowers will work, but you'd need to plant them later than usual so they don't bloom too soon.

Here, perennial phlox bloom right into September, but I don't know if they do there.

Hopefully, someone in a climate more like yours will jump in, maybe Trish?

Jeanne