Colorado is a fantastic state, I've driven some roads there from Denver and Boulder into the Rockies. Absolutely beautiful. A lot of it is rolling hills, a bit like Tuscany.
It is a dream of mine to ship my 330GTC over to the US one day, and hopefully drive coast-to-coast. But I think it will be a long time before that happens. It's a very expensive and time-consuming thing to do, and there are so many other things on my list as well (like visit Antarctica). But undoubtedly it would be amazing.
There are a few well organized trips and many wonderful roads in both the Western and Eastern US. However the concept of driving coast to coast is over-rated in my opinion. Much of the journey would be on flat, straight and seemingly endless interstate drives that are more drudgery than entertaining.
Check out Vintage Rallies. The Taylors organize these trips around fun roads, great scenery and nice resorts and dining. I would not waste my times traveling through the flatlands in the mid section of the country to get to the other coast.
Current: SA, 330 GTC, Dino, BB, QP
Past: GTC, BB, QM, C/4, 355, 360, 550, 575, C/4