Just visited family in Detroit last month, drove up North and back down through Milwaukee and flew out of Chicago.
We spent the day in Milwaukee. Other than sports, beer, sausages and Harley it's not the center of the universe. BTW, I have been to Milwaukee half a dozen times over the years (first pic is my '62 Chevy at the Harley factory 20+ years ago). Trip Advisor is a decent travel site and ranks the top rated "Things to do" in nearly every City in the World!
Wow! I thought the Harley museum was amazing! Seriously! I did the factory tour 20 years ago and figured we would do it again. Come to find out that Harley moved their engine plant out to Menomonee Falls and built an incredible museum! We spent a good 4 hours rushing through, probably should have spent all day. It's multi level and very interactive. I want to say it was around $20 bucks to get in. We were there during a bike ralley (Labor Day I guess), so there was a LOT going on outside.
Chicago is 90 miles South and one of my favorite US cities, so it's worth it if you have time. When I do the tourist thing (or take someone that has never been to that city), I do the "Hop on, Hop off" tour bus. It's usually between $25-$90 a person depending on the city. I think the one in Chicago was around $25 per person. It's nice for a day (especially if you stay in the city), to listen to the tour guide... hop off, grab something to eat or go to a museum, etc, hop back on and go to the next attraction.
Hope that helps.