All time favorite- Long Trail Hiking in the fall
Boy, do I ever miss living in Maine when the Fall Foliage report start coming in!  I spent my grad school years living in Northern New England and I don't think I studied a single minute between late September and late October. Without any sort of direction or guidebook, I was usually found driving my car down unmaintained logging roads and bushwhacking random summits in search of the perfect Fall Foliage experience. I might not remember everything I learned in class, but the view of a fall forest in full “bloom” is something you never forget.
Doing online research about how to see the best fall colors is like the proverbial experience of “trying to drink water from a fire hose”. Most of us seasoned leaf peepers have our own tricks and tips that we typically keep close to the LL  Bean vest. But now that I live far away, I feel like I should share at least some of my knowledge! So, here's a few of my thoughts on the subject that I didn't typically see online or in a guidebook:
Get far from a paved road: Nearly every guide about leaf-peeping that I have viewed is mostly based on what you could see from the window of a car. Most leaf peeping reports are based on observations from highways and popular drives. However, I've always found the best foliage viewing comes from the further you can get from a (paved) road.  So even if you are not to keen on hiking 10 miles into the wilderness, if you have a four-wheel drive and/or some comfort with traversing partially-maintained logging roads, you're going to have a better experience.
Elevation makes a huge difference
Elevation: A less-experienced leaf peeper will probably point their car in the direction of any mountain range New England expecting that the colors are just going to be fantastic no matter what. Certainly the mountains can be gorgeous, but you're going to find more evergreen Spruce-Pine forests at higher elevations. Don't get me wrong, pines add the right amount of variability to the brilliant oranges, reds and yellows of oaks and hardwoods. But a mostly boreal forest in the fall looks just about the same as in winter, spring and summer! This is a broad stroke, but my favorite fall colors in New England tend to range between 1,500 and 3,000 feet  where just a few of those evergreens can create a magnificent sight alongside the real stars of the Fall show.
Use the State and Local Tourism Guides: This might be a bit of a self-defeating point given that I'm writing this from California, but the most reliable source of accurate and up to date leaf peeping reports comes from the most ultra-local of sources. Most of the more big-name websites often have broad, same-every-year reports on fall foliage. You want to know what's really going on up in New England? Start with the state tourism websites which keep a constantly updating report from people with their boots and tires on the ground. They are going to be much more sensitive to the micro-climates and subtle fluctuations in weather that lead to the best days to go out. If you know exactly where you’re going to go, try and narrow your search to regional, county and city tourism websites that typically also produce highly accurate reports on foliage.
Luck: If there's one thing that plays the largest role and leaf peeping in New England, it is sheer luck. Fall color is a combination of plant biology, meteorology, regional variations of climate and  elevation. Sometimes all these things come together and you happen to be in the right spot for the most fantastic foliage. In spite of all the planning and knowledge one can acquire about the subject, there are other times when the trip is a washout. You can use luck to your favor by broadening the area that you cover on a trip and trying to take multiple trips throughout a season.
My favorite areas?: Ah, I knew I would have to divulge them someday!
Vermont: I can't say I was ever disappointed by a well-timed trip to the Northeast Kingdom of Vermont. Specifically, Newport, Vermont is where my fondest memories of Fall Foliage come from. Lake Willoughby and Willoughby State Forest off so come to mind as fantastic places to be. I have also considered the Route 100 drive through Central Vermont to be the automobile-equivalent of the Long Trail. One would not be disappointed driving this road any time of year. There's enough variability in elevation that it is hard to imagine not having a good fall color experience. Emerald Lake State Park, Mount Dorsett and Mount Equinox are all so dear to me from my many adventures through the state.
Maine: Sebago Lake and Long Lake near Naples Maine are close enough to Portland to draw some substantial crowds on the weekends. Nevertheless, there are some back roads and and short hikes that retain their rural character. State Route 113 from Standish to Freyburg is almost constantly rural but stays at low elevations along the Saco River  allowing for effortless for viewing. The hike up Cutler Peak near Hiram is easy enough to be done in an hour yet I've never encountered many other hikers on the trail. Caribou-Speckled Mountain Wilderness Is typically passed over by the hoards of hikers seeking the higher summits of the Whites but serenity and solitude are in abundance near Shell Pond and the Royce Summits. The towns of Rangely and Eustis, Maine are also destinations that I would go to any time of year. What draws me in the fall is the alpine lakes and chance to hike through some very remote wildernesses abounding in autumn hues. Lastly, Moosehead Lake can be a bit difficult to time as far as the season goes, but it remains far enough away from any major center of civilization that fall color drives here are rarely crowded.
New Hampshire: Again, most hikers and casual drivers are going to be centered around the White Mountains and Mount Monadnock for their fall experiences. I never really encountered too many Leaf peepers hiking through Mount Kearsarge State Forest, spending time around Newfoundland Lake and hiking Mount Cardigan. Driving what I call the “Connecticut River Corridor” from Bethlehem to Pittsburg is considered far off the Beaten Track for most casual explorers but there are plenty of Bed-and-Breakfasts and small hotels that bring you close to creature comforts.