We love the area around Lake Itasca/Bemidji - camped at both parks many times when our kids were at Concordia Language Village outside of Bemidji. The headwaters is an outstanding place to visit, you'll get a special kick being from STL to be able to walk across the river. If you have bicycles there are great trails at Lake Itasca but your schedule may be a bit tight, it's a huge park with lots to see. Park Rapids and Dorset are neat little towns south of Itasca if you have time. One other recommendation - on your way from McCarthy Beach to Cascade River try to stop at Soudan Underground Mine SP.
Soudan Underground MineVery cool place, great tour of the old iron mine. Some years ago they were using part of the mine to house targets for particle beams shot from the accelerator at Fermilab near Chicago as part of Tau neutrino research, not sure if that is still going on.
Also in that general area is Hibbing, home to Bob Dylan's birthplace, the Greyhound Bus Museum, and the largest open pit iron mine in the world.