While not a concert, per se, if you can make it up to WIsconsin, you can drop in on Lake Michigan at several launches in the area, and head over to the Summerfest grounds. Depending on what you might want to hear, you'd have to move up and down the shoreline around there, but you might just find it easier and more enjoyable to tie up and walk the grounds - the main stage (American Family Insurance Ampitheater) is the only stage you'd have to pay extra for, if you wanted to see someone playing there. Usually, that's the current headliner type of act, Dave Mathews Band is supposed to be playing September 15, for example. All the other stages are free with admission to the festival grounds, and if you've never been, there's enough that you just might want to plan for a few days or even the whole week.
It's been pushed back to September this year, but that's better than not having anything at all, like last year, and hopefully next year can get back to its typical spot in late June - early July. The other ethnic festivals are fun as well, if you're into that sort of thing. I'm partial to German Fest and Polish Fest, but they've got a few others that are tailored to specific heritages and celebrating said heritage.
Summerfest presented by American Family Insurance is one of the most iconic celebrations of music in America, hosting the biggest acts.
www.summerfest.com
Not sure if they're doing anything yet at the Leach Ampitheater in Oshkosh, WI, but if they bring back the concert series, that would be something to come to. Park on the water right next to the ampitheater, and there would be zero problems hearing whatever band is playing (I've seen Styx there, and just missed Ted Nugent and 38 Special due to my schedule, for instance). Thus far, nothing seems to be going forward, but that's something I keep an eye on, since it's only a 40-50 minute ride from my place.