You HAVE to get up to Northern MN. I'm always so sad to hear about people who come to Minnesota and never get past the MOA. Our North Shore (north of Duluth) is spectacular. Not just the views, but you can do hiking, biking, visit the state parks along the shore - especially Jay Cooke (just south of Duluth) and walk across the swinging bridge, Gooseberry Falls where your GS can climb on the rocks and see some spectacular waterfalls, Split Rock lighthouse where you can get a tour of the lighthouse area and look out over Lake Superior. You could travel all the way up to Grand Marais and get out on Lake Superior, drive the Gunflint trail and see Moose. Near Orr, Minnesota there is a bear sanctuary where you can see wild bear in their habitat. And of course, Itasca State Park has a ton to do. Walk across the Mississippi where it starts, take in a evening program, take a boat ride on the lake, bike the wilderness trail or just relax in the beautiful northwoods.
If you can't get that far north, at least drive up just north of Stillwater (about an hour from the MOA) along the St. Croix river and check out Taylors Falls. There is a great park where you'll find rock climbers, trails and you can do a river cruise or your GS could go to the water park just up the road.