Get high... in elevation I mean!
Any mountain town between 6000'-8000' will have fantastic summer/fall weather. Mid-eighties during the day, cooling off to forties or fifties overnight. Summer thunderstorms are a bonus :)
A few ideas off the top of my head: Flagstaff AZ, Stanley ID, Lake Tahoe CA, Jackson Hole WY
Lower in elevation but further north: Coeur d'Alene ID, Spokane WA, Bozeman MT