You have a couple of options. You could cut over through Southern NH and Vt - Rt 101 through NH, you'd go through Peterborough/Monadnoc Region, then on through Vt on Rt 9 and then go south to Lenox. Or you could take the more northern route through the Lakes or White Mountains. In either case I'd spend at least one night, probably two.

Then going back east from Lenox you could just take the Mass Pike but there are any number of scenic routes through the Berkshires to the Pioneer Valley (Conn. River Valley, Northampton and Amherst). You could spend one night in that area - Northampton is a cute town with Smith College, Amherst has three more colleges. Short detour is Deerfield with Historic Deerfield and also Yankee Candle. Then from there to the Cape I would go the Mass Pike but if you want to break up the trip Sturbridge Village is good - a living history museum. Also closer to the cape is Plimouth Plantation. A twelve year old should like any of these places. Google any of the places I mention for their websites.

I would plan to spend more than one night in the Berkshires, near Lenox. There is a to see and do...though it does depend a little on when you are doing. You and your daughter might enjoy Hancock Shaker Village, the Rockwell Museum, Chesterwood (the tour of the studio where Chester French sculpted the Lincoln statue is pretty cool). Plus if its summer, a chance to see the Boston Symphony at Tanglewood. Tanglewood is the best place to introduce a child to classical music, because you can picnic on the lawn, and she can get up and walk around and explore the grounds if she gets restless. Or read or draw, if she gets bored.

Consider flying into Albany, renting the car, passing thru Lenox on the way to the Cape. Then north to Boothbay, back to Portland, drop the car and fly home from there.

No backtracking and it gets the obligation out of the way at the beginning.

You might want to spend two nights in Portland to give yourselves a full day there. Visit the MDI Ice Cream store on Exchange St and get a sampler of their most unusual flavors. Get french fries at Duck Fat. Go to the bakery for the best whoopy pies. The mailboat cruise is fun. Your 12 yo can see how a kid living on one of the islands commutes to school.

I agree that cutting over to the White Mountains on your way to Lenox would be a nice detour. There are a number of tourist attractions such as the hike the The Flume. I guess everyone should visit Clark's Trained Bears which has been around forever. A more educational spot would be the Squam Lakes Science Center in Holderness. A daring 12 yo might want to try one of the zip line places. If your visit is in August, you could go over to northern VT's Great Vermont Corn Maze in Danville.
From there you can drive south to Lenox in a few hours. Probably all day if you stop for lunch and another activity. Both VT and NH have maps to cheese, ice cream, wine, etc that you might use to get off the main roads. Ben and Jerry's ice cream tour could be a stop.
It all depends on your interests and where you are coming from. The mountains might be a big deal and a lot of fun. There are places where you can swim and play in the water in a rocky river. We like to do picnics at places like that. Once you are here or have precise dates, you might search for local events that suit your interests. Tanglewood is world class but there are also small outdoor concerts in various New England towns during the summer where people bring a picnic lunch and enjoy music on a lovely summer evening.

One site for information on things to do in the area is

www.visitnewengland.com

Thank you so much for all your insider knowledge. We will now be able to plan a full and interesting holiday. Mt niece is working at the Bellevue summer camp and has mentioned Tanglewood. I think Morgan Freeman did an introduction there last year. I will check out their programme. My daughter is adventurous and will love the zip wires and Ben and Jerry's...what's not to love. We will be there middle to end on July. Looking forward to it...lots.
Thanks again for taking the time to help out.

Gillian

Finding plenty to do in the region is not your problem. You are going to spend far too much time in the car, even in this small part of the country.

What you are essentially doing is driving from London to North Wales, say, by way of Edinburgh. Would that make sense?

I would save Chatham for another trip and do: Boston, portland, Boothbay, White Mountains, Connecticut Valley, Berkshires, Boston.

It will not be simple finding accomodation near Lenox at this time of year. It is the 75th anniversary of the Tanglewood Festival, and lots of famous performers will be ddrawing crowds. You need to start looking right now. Lenox will be very expensive. The next town north, Pittsfield is not glam but has most chain motels. For a bit of real iconic Americana, you cannot beat the Red Lion Inn in Stockbridge, the next town south of Lenox. Prices are often quite reasonable, esp midweek. You may also run into two and three night minimums in Boothbay and you certainly would most places on the Cape. This is the height of the tourist season.

I'll stop hectoring but can offer more advice if you want it. I live on an island off the Cape, visit friends in the Berkshires often (she is a member of the Tanglewood Chorus) and have visited both the White Mountains and Boothbay Harbor for many years.

To add to your choices - while not exactly between Maine and Cape Cod, Rhode Island might be of interest to you. Providence is worth a night and then head down to Newport and some of the other coastal towns. Different type of shore than either Cape Cod or Maine.

And agree, add a second night to Portland.

Some great suggestions - thank you all very much.
Gillian