Sorry but I would not spend my summer in the Midwest (I lived 20yrs in MN). High humidity and bugs make it miserable. In fact I was there early July, had to get back to AZ where it was cooler (when you factor humidity) and less bugs. That area is great in the spring and fall though.
Best bet would be anywhere in the western mountains. Cool temps, low humidity, and not many bugs as long as you stay away from wet area's. CO is probably the best as it would be the closest for you but why settle on one area when there is so much to explore in CO.