THanks for the comments. This is what I had feared about this sort of route. We normally do a good 3000-3500 miles per trip and although the odd day spent on the Interstate is par for the course as you say the scenery on the West Coast more than makes up for it. I'm really into big scenery, hence really wanting to Colorado etc, but she wants to see more of Canada and visit Niagra Falls.
The Applachians are very interesting, definately taking in at least Mount Washington and happy to detour further South on the way back from Chicago, is there decent views/scenery to be found by doing that?