Sedum 'Autumn Joy' looks great in August. So does PeeGee Hydrangea--- it's also cold-hardy in your zone and can take full sun. Rudbeckia (a.k.a. Black-eyed Susan). Your coneflowers and Asters should still be blooming then.

Well, that's a start, at least.

Due to lousy weather & a shortness of time, I, too, got a very late start planting my flowers. At this time of year you may find garden centers, especially the mom & pop types, very anxious to get rid of their plants. Due to my late start, I purchased larger plants in 6-inch containers, such as wave petunias and other types normally used in containers, and put these in my garden instead. The Proven Winners series are especially good for long blooming periods. These will fill out nicely in no time at all. I'm sure if you hunt around, you'll find some good-sized plants at bargain prices.

Just a thought...I was touring the grounds, admiring the lovely flowers, at a beautiful casino down in CT one time. Upon closer inspection I found that they put the flowers, pot & all, in the soil, hidden by mulch, for a quick flower display!