This Saturday is the Smithsonian Magazine Museum Day, which gets you free admission to a lot of museums across the US. You do have to pick out a museum and print off certificate in advance of going. You must have this certificate in order to get FREE ADMISSION.
There are 18 locations in Minnesota listed. In the past the Minnesota State Historic Sites had participated in this program, but it doesn't look like they are anymore (insert my sad face here). This weekend we won't be near any on the list, but if we in the Twin Cities or our home I would check out the Minnesota Landscape Arboretum, The Works Museum (kids science museum) or the Steele County Historical Society Museum.