Too bad you can't add a day or two. The middle route using Interstate 70 offers up some great stops: Gateway Arch, Truman Library, Eisenhower Library (short detour). Then south from Denver on I-25 to US Highway 160, then west (the route Chevy Chase takes to get to the get to the Grand Canyon. On the way you'll see Great Sand Dunes, Mesa Verde Nat'l Park, then thru Navajo reservation (think of Forrest Gump ending his run across the country) to the Grand Canyon. Then on to LA.