It helped to have spent time in QC before, so we knew where the highlights were. Tip: walk through the lower city and either take the stairs or the funicular up to the upper city to get to the Plains of Abraham and the man city. There is also a nice public market a few blocks east of the ship port.
All of our group enjoyed PEI. One took the Anne of Green Gables tour, another couple spent time in Charlottetown, finding it fun and charming, and others rented a car and toured the middle part of the Island. PEI seems unspoiled, really a nice place whether you stay in town or visit the shore, historic sites, etc.. Tip: go to the visitors center at the museum, a few short blocks from the port - the staff was very helpful and there are computers available.
There's not a lot to do here, but we enjoyed walking around the town and taking care of a few personal errands. The bid draw here was the Alexander Graham Bell home and museum, which means either a shore excursion or car rental.
Halifax is sort of a smaller Montreal: a major city with everything from churches to large sports facilities. We headed for the boardwalk and Maritime Museum, a very interesting experience. We also walked up to the Citadel, which costs money to enter - but one of our friends got in free by walking around to the north side. The Public Park near the Citadel, Victorian in vintage, is lovely and peaceful.
In some ways, Bar Harbor is enjoyable just because of its setting. Some of our group walked the shore trail, some took an excursion to the Acadian National Park, some shopped. We ate lunch (clam chowder, a huge haddock sandwich) at a tavern-restaurant in town called the something Whale. Bar Harbor did seem to be the more touristy port we visited - but a lot of the tourists were not from cruises. They were traveling on their own, by car or other means.
I can't believe I'd never been to Boston. There is a lot to do here. We ended our trip by staying in a short-stay apartment for 3 days. I recommend the Museum of Fine Arts, the North End (especially to eat), the Common, Faneuil Hall/Quincy Market - and getting around on the T. You can get either a CharlieCard (plastic) or CharlieTicket and add money to it as you go along. But Boston is also walkable - we walked the entire Freedom Trail, including crossing the Charles River and visiting the U.S.S. Constitution, in part of one day.