If you're looking for dramatic postcard-type landscapes, Superior is probably your best bet -- but Minnesotans see so much of it that we're a little blase about it (there are several photographers who support themselves fulltime with Superior and North Woods-based photography, and we are inundated with their stuff.) You will also waste several hours driving back and forth from Duluth; it's about three hours each way. Duluth itself is very interesting if you're into industrial landscapes, like that around the port. Go to the Radisson Hotel's restaurant on the top floor for some spectacular views. The drive from Minneapolis to Duluth, on I-35, anyway, is one of the duller trips around.
Another good option (better, in my opinion, but I might be suffering from Superior overload) would be to get a map and drive a few miles east of the Twin Cities to the town of Stillwater, spend a little time there (maybe take a padddleboat ride), then work your way down the St. Croix River to Prescott, Wis., where the St. Croix runs into the Mississippi. Stay on the Wisconsin side down the Mississippi hitting the little towns along the river route, until you get to the south end of Lake Pepin, then cross the river and turn north and follow the Mississippi back to the Twin Cities, stopping to climb Barn Bluff at Redwing on the way back. Look at the bridges as you go. The whole trip would be fewer miles than driving one-way to Duluth, and you'd see a different (and historic) landscape. You could add to the experience by reading Mark Twain's "Life on the Mississippi" before and during your stay. Uh, if you're going to be out at night, or if you're going up Superior's north shore, bring a jacket; and if you're going to be on the river, some insect repellent.