As others have noted, parts of upstate NY can be pretty cool during the summer. Adirondacks are the first thing coming to mind for those parts.
Personally, we like the mountain towns in the Rockies during the summer. Estes Park, just outside of Rocky Mountain National Park is one of my faves. Anything outside of Yellowstone and Grand Teton, if you can find lodging, is also pretty nice. If Grand Teton, I'd focus on Jackson/Jackson Hole. If Yellowstone, most people opt for West Yellowstone if staying outside of the park, but I prefer Gardiner. Cody is also an option, but Cody gets a little warmer and Cody is more of a drive in and out of the park. Anything around Glacier in Montana should also offer nice temps, but it books up really early so I'd get right on it if that's of interest. Crossing the border into Canada and looking at Banff and Calgary is also a thought.
I was just going to mention Estes Park. We loved it there and with the higher elevation its cooler.