AGB said this: you could do a loop up to Itasca, across to Soudan, and then continue to the North Shore (check Gooseberry Falls S.P.), then back down through Duluth and maybe pick up the Great River Road as you head back south.
I totally agree. Also, include Bemidji and pay the statue of Paul Bunyan a visit - so fun! The story of Paul and Babe the Blue Ox is total fun. When further north, include the Hull Rust mine - lots of fun activities for kids there too. If you can... drive all the way up the North Shore to Grand Marais - both there and at Split Rock Lighthouse you'll find some fantastic living history. (Visit the lighthouse keeper's house at Split Rock and, if you're lucky, hot chocolate chip cookies will be coming out of the wood stove oven as baked by the keeper's wife). Walking across the Mississippi at Itasca is a MUST do.
Bring your bug spray. We got a refrigerator magnet from way up North a couple of years ago... it shows a huge mosquito, napkin around its neck, holding a knife & fork in its "hands" with the caption reading: "Send More Tourists, the last were delicious!". Deep Woods Off seems to work well.
Enjoy!!