When I did it, about 20 years ago, I started in Boston, drove up through Maine (stopping overnight at Portland and Bar Harbour), crossed into Canada and followed the main road through NB, into NS and, once again followed the main roads around NS in a clockwise direction. Then drove back to Boston for flight home to UK. (PEI was a different trip). Really the point is that, follwoing that route, I never had to deviate by more than about 20 miles off the main roads to find brilliant photo opps.
Of course, that trip took about 2½ weeks. If time is short, then my suggestion would be to fly to Halifax and drive north to Cape Breton and spend most of your time there. Lots of great opps round about Margaree Harbour, Chetticamp, and through the national park to Sydney.