Summers are hot and humid across much of the US so I would head for the hills! The Rocky Mountains National Park in Colorado, Lassen Volcanic, Yosemite, Kings Canyon and Sequoia National Parks in California, Yellowstone and Grand Teton National Parks in Wyoming, Glacier National Park in Montana, Crater Lake in Oregon, and Mount Rainier and the North Cascades in Washington are all great places to visit in the summer as is Alaska if you want a real adventure.
All the roads and park facilities will be open and you have the best chance of pleasantly warm days and mild nights.
See our National Parks section for more cooling inspiration