The forests and mountains outside of NYC and Boston will give you a tremendous range of colors, October is usually the best month.
I'm a little prejudiced about Cape Cod since I lived there for 5 years, one could spend 2-4 weeks there alone to really see it all. The light on Cape Cod, especially in late Sept and through October has an indescribable quality that makes the blend of sea, sky and land the makes up the cape, especially at the easternmost end, feel like a magical place. I lived in Provincetown, located at land's end on the cape, and would hike or take a boat out to the very tip where the lighthouse is located. With the Bay on one side, and the Atlantic Ocean on the other, you can see the sun rise and set over water, and a stormy day makes for dramatic waves. While I lived there, I di not own a car, walking or baking was very easy in such a small town. Even now when I visit I take a bus or boat from Boston, it leaves me more time to enjoy the scenery and not have to concentrate on driving. Further west or "up cape" there are many nice towns like Chatham and Well Fleet, and Nauset Beach is a great location for towering dunes and crashing waves on the Atlantic side. On the edge of the Cape Cod Canal, at Falmouth, one can walk along the canal and often see large ships transiting from Boston to southern ports. All the Cape Town date back to the early 1600's so there Is a lot of history, and a unique architectural style in many of the old houses.
As for travel between Boston and NYC, it is the one place in the US where train travel is practical and reliable, and travel by train between the two would be much more enjoyable and probably faster than fighting traffic usually is between the two. You could also travel to Philadelphia, Baltimore, and Washington DC from NYC by train in about 6 hours All those cities have much to recommend seeing for their history and architecture as well. If however, you were driving out of NYC , you could go to the Jersey Shore and down to Cape May, which are fantastic to visit in the off-season, most places are still open, but far fewer crowds, and the pace is slower. In September-October, New Jersey also has a lot of fall color, and many beautiful areas from mountains to ocean. The problem is, that is not what you see when you drive through the state on the most direct route ;) Anyway, I lived there as well, and I love hiking in the Ramapo mountains and visiting High Point park. I have family in New Jersey, I enjoy visiting all the places only the locals know, and eating the best food in the world, from diner style to real Italian, to fresh pierogies. No wonder I have to hike so much when I am there.
Another option from NYC is to take either train, or the Day Liner boat up the Hudson River. From the Boat you would see the same sort of landscape the inspired the Hudson River school of painting, absolutely breathtaking, with incredible fall colors. IF you drove up Route 9 along the Hudson, you'd pass through dozens of lovely small towns, and on the west side of the Hudson river there is Bear Mountain State Park, with its restored carousel and huge ski lodge near the large lake. You could continue north through NY State,, then turn east into Massachusetts, or into Vermont, and head east through the most storybook landscape of fall foliage, and continue back to Boston.
IT 's too bad the winter weather is rapidly approaching, I really feel the need for a road trip now. I live in Virginia, south of the areas I described, but no less interesting and beautiful, especially in spring and fall. Come summer, the heat and humidity can be a bit much, but at least it's very green.
