I'm not sure how many of the people that read this blog are in a position to go on a demanding four-day hike in the mountains of New Hampshire, but if you are I can't help but recommend this circuit my father and I discovered in the fall of 2006.
We took a four day weekend around Columbus Day and decided to wander all over the Presidential Ridge, often described as one of the best part of the 2,175 mile Appalachian Trail. We tried to maximize the amount of time spent above treeline since you get some of your best views there. We also tried to minimize the amount of duplicated trail -- no point in hiking the same stretch of trail twice if you can help it.
Click on the map to see an interactive version.
The terrain we picked varies from really quite steep at times (about a class 3) to nothing more than strolling along alpine meadows. The varied landscapes take you from the exposed ridge tops to quiet river valleys. The only real constant is that you'll be putting in around 10 miles of serious hiking each day, which is at least twice as far as the average weekend warrior prefers to do. That being said, pretty much all of the active people I know who read this blog could do it and enjoy it (I'm looking at you, athenaHuck).
(full set of photos from that weekend are here)