Continuing OP's trend of Western US cities, Portland OR is very nice in August. The city offers many sights that are walkable-- easy walking, not strenuous. The light rail and taxis/Uber offer good fallback options. In addition to spending a day or two seeing downtown's many sights, spend a day driving the Columbia Gorge, a day driving out to the coast, etc.

If OP wants to stay a bit closer to home (2 hr flights from ATL instead of 6) I second the suggestion upthread of the NY Finger Lakes region. Summers are beautiful up there. Warm days but rarely hot, pleasant evenings that seem to stretch on forever. The area is very driving oriented but is dotted with numerous small parks that offer a mixture of easier and more challenging walking trails.