OP, we have been to some of your potential ports several times (and some not at all), and I understand that you are overwhelmed with all the research. However, especially regarding Portland and Halifax, my advice would be to NOT do an excursion. Both are attractive cities with interesting histories. Both lend themselves to just walking. In Portland, you can take the mail boat, the narrow gauge railway, walk up a street of Victorian houses to a nice lookout. If you want to see a lighthouse, get up early, before your ship docks, and watch the ship sail past a lighthouse. Portland is also known for its food and breweries. In Halifax, there is a waterfront boardwalk, with the Maritime Museum several blocks from the port, and eateries and pubs along the way. Walking up the hill will take you to the Citadel, botanic gardens, old cemetery, and more. We always enjoy just rambling in both of these cities, and never understand why people book bus tours out of town. Town is where it's at!