I would probably stick to the fastest route going in, but then maybe take a longer route going north on the way back? So maybe take I-15 from Las Vegas north through Salt Lake City and Idaho Falls and visit Yellowstone National Park. Then swing back east and connect onto I-90 to go through the Black Hills National Forest and Mt Rushmore, then to the Badlands National Park on your way back to Minnesota. That gives you a nice fast highway on I-90 for a lot of the less scenic driving, while hitting some really scenic stops on the way out. Going in, if you are heading down to I-80 and I-70, you can always do some small detours to Pikes Peak or Grand Canyon National Park.
