I'm not sure what kind of distance you're looking for, but the Katy Trail in Missouri is a favorite of mine: http://www.bikekatytrail.com/. At 237 miles, it's the longest rail trail in the US.
Most of the ideas I have in mind have been mentioned. I will reiterate that although beautiful and great to bike in, it can be difficult to find places to stay in the Finger Lakes of New York. Places book up pretty fast. I would like to suggest Vermont and Northern New England. That area has all you have mentioned. You can also cross into Quebec, which has one of the finest cycle route networks anywhere. Colorado is stunning. My only reservation with recommending there is that if you are not acclimated, the air gets very thin. While in the high Rockies, the valleys are at an elevation often higher than the high mountain passes in the Alps.
Circumnavigate Lake Champlain?
Originally Posted by professorbob
Originally Posted by briwasson
That is a good idea. Here is a Northern California tour. It'd be pretty easy to figure out what this route is exactly, using the Strava Route planner with Heat Map.