Some ideas:
A loop in Colorado
Montana + Canadian Rockies
SF->LA
SW France (e.g. Bordeaux to Toulouse via the Dordogne/Lot valleys, then south from Figeac to Toulouse)
Corsica
Pyrenees
Personally, I found the flat Po valley in northern pretty boring relative to the rest of non-flat Italy. I also didn't find the Ardennes particularly exciting.