It's hard to pick just a couple, here's a few areas that you might like in Minnesota...
-Duluth & the North Shore
-Itasca State Park and the headwaters of the Mississippi River, and the surrounding areas - lots of lakes
-Mississippi River towns of Southeast MN such as Red Wing, Wabasha, LaCrescent
-Open prairie of Southwest MN and Native American history at Pipestone National Monument (a good stop if you're headed towards South Dakota)
If you come to Minnesota from the south or east, you could do a tour and hit all of these areas, then head on to South Dakota. It would go River Towns, north to Duluth, North Shore as far as you wanted to go, then west towards Itasca State Park area, then southwest towards the prairie/Pipestone area. Then west to SD.
Have a wonderful trip!