I did a three week driving trip a good few years back, but I also go to Boston for work a lot.
We visited Maine because I'd read so many cheesy Stephen King books I wanted to see the places 
Generally the best time to visit New England is for Fall due to the autumnal colours and foliage.
3 or 4 days in Boston - follow the walking trails, go whale watching, take in the city, out to Havard, Bunker Hill etc would suffice. Salem is a pretty interesting stop - to learn about the witch trials, Kennebunkport also.
Maine has some lovely lakeside stops and Ogunquit beach is quite nice.
I enjoyed Rhode Island, Martha's vineyard and loved Cape Cod - hoping I can one day get a month off and rent a beach property there with DS a la Amity Island!
I bought a Fodors guidebook to New England, you can google "Drive New England" for some road trip ideas too. Virgin also give you good routes and hotel tips?