While more crowded than other times, northern parts of Michigan or Wisconsin would be lovely. Michigan's Upper Peninsula may be crowded, but like you said--World Cup. A flight into Chicago could easily get you to anywhere reasonable in Wisconsin via car hire (Madison--not sure how crazy summer is though). Michigan is one US location that has good train service. Beautiful views and climate. Home shares also could be easy in these areas due to uni professors taking holiday. You're likely familiar with those sites. Oh, and adding to the Minneapolis suggestion, look north as well to Duluth. Beautiful city, Might be a perfect place with proximity to much more.

I don't blame you one bit, by the way. One of the cities I telecommute from much of the year is on those host cities. Today I learned that despite having basically no feasible public transport in the greater metro area, there will be two "drop and rides" for the airport, which are "free." It's a trial approved by two cities to operate in light of the World Cup coming... Paid for by the hotel tax (whioch I just bet they raised--and it's pretty high in both cities). It's going to be chaos. I am sure it will be fun for those who love it, but not a quiet family getaway! Not that I expect anyone would go to the WC for that!