Another plug for the Pacific Northwest, in particular Oregon. Though it's recently been 100 degrees, summer is usually mild, especially at the coast where it can be 60 degrees when the rest of Oregon is hot. "Crowds" in Oregon are not epic compared to other places though if you really want to be away from people you can do that too. Car travel required - alas, our public transit sucks other than the Portland metro area.
You could fly into Portland (PDX) and head to a coastal city to set up camp. Astoria (2 hour drive) or Newport (3 hour drive) would be my picks. You could also set up a base in a city just outside Portland (e.g., Gresham, Wilsonville, Newberg or McMinnville). You'd be an hour or less from plenty to do - Portland city activities, food, wine country, breweries, state capitol, historic areas like Champoeg, plenty of state parks, Mt. Hood.
Or you could fly into Bend/Redmond area - it's high desert so it's "hot" but the heat is dry and it cools off at night. Plenty of outdoor activities, breweries, and our only national park about 1.5 hours away (Crater Lake). Within an hour are fun towns like Sisters (which is pretty crowded in summer, but still worth a stop).
There are plenty of ideas on the Travel Oregon website, including itineraries. Good luck!