I have a nice greenhouse near me with a huge selection of plants that are hardy to this area. I have found it particularly helpful to visit the greenhouse at different times of the year, especially August into September when most plants have seen their prime. Hardy mums will bloom twice, once before July 4th and if you cut them back, again in September. A nice companion to peonies, and coming in a variety of colors. The University of Minnesota has released some really nice hardy types that have lived here for years. Rudbeckia is another later bloomer. Sprinkling seeds of annuals like the tall zinnias are beautiful and colorful as well and are very easy to grow from seed, sown directly outdoors after frost window is past. They bloom from July to frost. Anemones are lovely as well.
Lilies, daylilies, delphiniums are others that come to mind. The list is endless!