My SIL goes to most of the ethnic festivals that go on there all summer long, and she really enjoys the Irish one. Don't go in expecting WDW , but it has good drinks/eats/music/entertainment. It's very pretty right there on the lake, so just the location alone is pleasant at this time of year. I've been watching the weather since I'm going to be in Racine county for a family picnic on Sat afternoon, and it's calling for 30% chance of thunderstorms, so you might want to pack a coat or umbrella.
We used to live in Milwaukee and we enjoyed going to the festivals - especially the ones at the lakefront. The weather should be nice this time of year, and you can expect good music, food and more.