I have not driven this way, but I think one possible scenic route would go from
Seattle across to Montana on I-90 to
Whitehall (Montana), and then take US 287 down to
West Yellowstone. Then go into
Yellowstone National Park on US 191, down through
Grand Teton National Park and all the way down to
Rock Springs. From here you take I-80 over to
Laramie and the US 287 down to
Fort Collins and then I25 down to near
Broomfield
