The White Mountains
What can I say, I want to go back! The Kancamagus highway should be near the top of your list of places to visit in the Fall. Expect lots of traffic and a slow ride, but there are plenty of spots to turnout and enjoy nature's show. Avoid North Conway on the weekends, as you could mistake the traffic for something you'd find in Boston.
Franconia Notch
Unfortunately, one of the highlights of this area - the Old Man of the Mountain - is no longer with us, having fallen victim to erosion.
Waterfalls
Do yourself a favor and pick up a book - the one I found was Waterfalls of the White Mountains. Although lacking pictures of the falls, it does provide hiking information - which you need, because many of the falls in this area are not easily reached with a short walk from a parking lot.
Shelbourne Birches
A small pull-off on the road between Gorham, NH and Bethel, ME, I almost drive by the first time I went looking for this birch stand. A good area if you're looking for the paper white bark framed against some colorful backgrounds. This area is being selectively thinned as part of a management effort so when I stopped by there was quite a bit of scraps leftover from the cutting process.
Mount Washington Hotel
The elegant old lady of the presidential range. Pricey, but you only live once right? Situated at the foot of Mount Washington, you walk the hallways and find yourself drifting back in time. Spend a few moments in front of the main fireplace, or take in a string ensemble in the Conservatory.