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.
