Don't come to Iowa!., at least if you want nice weather!
However, I have thought of a few less well-known places that I have spent summers in that average temperatures in the 70's.
Flagstaff, AZ and the beautiful Coconino Forest. There are so many interesting things to see and do outdoors. We spent many days hiking, my kids played in the streams, had picnics. The whole area is great, from Prescott to Jerome to Sedona to the Grand Canyon.
Erie, PA (specifically, Presque Isle) I spent a summer there when my then-husband was doing consulting work for a company. The small island is a national park, and had no entrance fees (I believe the whole state has free entrance to their parks). It was the least crowded beach I have ever been too, except on a few weekends when they had specific events. Often, no one else was there. My daughter was two, and we spent most days at the beach or at the small zoo. We took picnic lunches and sometimes ate at the small eateries which were pretty good. The water is cold, of course (Lake Erie) which probably explains why people don't flock there, but I loved it. Was one of the best summers of my life. They have boat tours, including tours of the swamps.
Big Sky, MT. Less than an hour from Yellowstone. It is a popular ski area in the winter, but in the summer the weather is beautiful, and they have outdoor music festivals and other outdoor events. We watched fireworks on the 4th one year from the front of our condo and it was spectacular. We have a relative that owns a couple of condos, so we enjoy going often.