My husband, best friends-C and VR, and I are going to Boston on October 2, then to NH, VT and Maine before going to Nova Scotia. We have never been to New England and would really like suggestions. We will spend 3 days (incl. day of arrival) in Boston and then head to New England. We want to spend at least 4 days in Nova Scotia and Cape Breton and will eventually end up in Niagra Falls and return home on Oct. 17. We would love suggestions.

Your timing is perfect for the beginning of foliage but unfortunately it starts in the north and higher elevations. It might be just starting in Boston but Columbus Day weekend is usually peak color in the mid-NH area. Keep in mind this is a heavy travel period so I would book weekend plans ASAP if you want to stay in any of the popular areas like Jackson NH. You could check Yankee Magazine's calendar of events for foliage festivals planned during your visit and head to one. You could take a foliage cruise of inland rivers from Portsmouth NH or one on the SS Mt Washington that cruises Lake Winnipesaukee in NH. From Boston you should head north to the White Mountains. You could choose a base, then make a day trip including a short hike or a visit to The Flume, a famous gorge. I like the foliage festival in Warner NH although we haven't been able to attend for several years. They have a parade, woodsmen events, tiny midway for kids, craft fair, farmers market plus it's a pretty New England village.
Or maybe head directly to Nova Scotia, with a stop in Portsmouth NH, and then back to Portland ME and then across to the White Mountains (about an hour from Portland). Continue thru to VT then to Niagra Falls. You might head to Burlington VT (fun city/college town) and take the ferry across Lake Champlain to NY.
One fun activity that we did (but in summer) was the Great Vermont Corn Maze in Danville VT. Plan at least 2-3 hours of walking. You might pit 2 against 2 to see if either team can solve the maze. Danville is not far across the border from Littleton NH which is just north of the White Mountains.
Perhaps once you get a lot of suggestions for activities and places to see, you can work out a map of what you'd like to do. You will be doing a lot of driving so please get out of the car once in awhile to do something and stretch your legs.

I like dfrostnh's idea of going to Noa Scotia first. It's a ver long drive, so would suggest the overnight ferry from Portland to Yarmouth, or travel up the coast of Maine and do a round trip on the Bluenose out of Bar Harbor. Spend some time in Bar Harbor to see Acadia National Park. Then take Rt. 2 across through Maine and NH (spending some time in the White Mountains) to Vermont. This will bring you the Northeast Kingdom, which should have some nice color by that time. Rt. 2 goes right through Danville, which was mentioned in the previous post. Drop down to Rt. 100 for a stop in Stowe. Continue on to Burlington, then take a ferry to NY and travel through the Adirondacks to western NY, and eventually Niagra Falls. That is a lot of driving! Are you returning home from Niagra Falls? Once you have your itinerary set you should think about making reservations. October is a very busy time in VT and NH. Would be happy to help with some specific suggestions when you decide on where you are going.

Dfrost is right as always and I might add a few things... First if you haven't booked your flight yet then you could land in Manchester NH's airport. that way you could drive north from there. around 2 October the color should be up in the Northeast kingdom to northern NH and northern Maine.
If you are going to follow her suggestion then go to Boston and drive up the coast to Maine and take the Cat over from Portland or Bar Harbor to NS.
Explore and then come back to either of the above places.
Then you can drive across Maine to Fryeburg and the fair there (Oct 3-10 2010).
Then take main road north to where it turns into state rd 113 and cross route 2 (route 2 was under construction last fall and should be finished by fall 2010 but I think you should head north to Rangeley lake area. following 2 east to route 26 (Grafton Notch stat park) then follow it north where 26 splits with 16. Here comes a decision, take 16 north and explore the Rangeley area or take 26 west to where it joins routes 3 and or 102. Take one of these south to the south west and from here it gets real hard.. there are no bad roads at this time of the year but 3 to 302 might be my guess and go through the White Mountain national forest.. or take 102 west and head in a westerly direction to St Johnsbury Vt and explore the Northeast Kingdom.
Now up to this point could be two days worth of driving or a weeks worth of driving. If you follow my tenets and get out of the car to explore it will slow you down and you will of course enjoy the trip more. There are tons of small towns and things to see all along the routes I've mentioned...

I hope this gives you some ideas to start with.

Jeff Foliage
www.yankeefoliage.com

and come west over route 16 and follow it

There is no Cat Ferry from Portland or Bar Harbor anymore. Service has been cancelled for the forseeable future. There is no overnight ferry either, that one was terminated 5 years ago.