This is a really cool topic for discussion. I personally live in Minnesota now, but grew up in Wisconsin. I've been meaning to visit the Yerkes Observatory in Lake Geneva, WI. I believe that parts of it are still active.
https://yerkesobservatory.org/
It looks like the oldest observatory in MN was one at Cartlon College.
The Carleton College Goodsell Observatory is fantastic to visit. About an hour south of the Twin Cities (Minneapolis-St. Paul metro area). they have public nights once a month much of the year. Check the website above. Very nice scope. Mechanical drive with the governor that is by itself fascinating to watch. It is well worth visiting.