Back to answer; these are things we have done or plan to July 25.
Boston - We walk down the street from the pier and take the Silver bus to South Station. Price is about $2 to $2.50 pp. From there, you can transfer (no additional charge unless you walk out of the facility like I once did by mistake). Take the Red line two stops to Park. Get out and you are right there at Boston Common and the Public Gardens. From there it is an easy walk to the Old State House, Faneuil Hall/Quincy Market, Union Oyster House, even the North End (cannolis at Mike's or Modern). Just check a map in advance for the other sights in walking distance. a two person tour for $10.00 or less.
As someone else mentioned, the Harpoon beer hall is two or three blocks from the pier.
Portland - There are reliable tours offered when you get to the street from the ship. I took one once for $27; I found it boring except for the extended stop at a great lighthouse that is or was a military site. Since then we have just walked the harbor area near the ship. Not bad on a nice day. Lobster rolls (my wife likes; I don't eat food that the Almighty has hidden under water and/or armed to attack). My favorite was walking a bit northeast (short walk) and getting a slab of Sicilian pizza from Miccucci's and taking it to the Shipyard store and brewpub a block away. Great flights of beer or soda for a good price, and they are happy to have you bring food in. Interesting store attached.
Bar Harbor - The number one choice is the Island Express FREE (donations accepted) bus system out of the town square park. Problem: The last day of service for the season is Columbus Day, October 14. If you are there, I recommend route 3 or 4. You can get off at any stop and catch another bus to move on. The longest I have ever waited was about 12 minutes. If not available, the previously mentioned Ollie's Trolley is a good choice. Book through their website and save big. If nothing else, walk around town including the shore path. It is a great walk.
Saint John - Our first time there, we walked around town. We found that dull. The three times since and our next in July, we rent a car. The Avis/Budget office is about a quarter mile down Water Street. Book through Budget; it's cheaper and the same cars. Lower rental taxes and the exchange rate, $41 American, unlimited mileage. We drive to the Fundy Trail (check it on a map). It is spectacular. Once we turned around because of fog. They, also, close some time in October. Check their website. On the fog day, we backtracked to the Sea Caves by the seafood shack on the shore road by St. Martin. A great stop. If the tide is right, you can walk into the caves. They are huge. And the rocks on the shore there - fascinating. We will go back this July.
Halifax has different options. One is the market next door and exploring up the waterfront. That was interesting. Got to have my first Tim Horton donut. Side note - Canadians eat more donuts per capita than any other country. We have also walked uphill through town to the base of the Citadel. That we enjoyed. I know Peggy's Cove is a biggie for people, but I can't justify the cost. Sooo, we have rented a car in Halifax. There are agencies within two blocks of the pier. I went with Budget, about $57 American. I'll have to walk a 1/2 mile to pick it up. They may have pick-up service; I don't know for sure. We will drive to Peggy's Cove and stop at Long Lake Provincial Park on the way back.
There is also a big grocery store nearby you can see from the ship. Very high deposits for cans and bottles.
And that's what we do! I'm not pushing any of them but wanted to let you know some things that are available. We like the itinerary, but have only done it from Baltimore on Grandeur.
I have done three Anthem 12 day cruises south. I like the ship. Book your three shows when available. Northstar and I-Fly were sometimes available for walk-ons. I easily made reservations at Customer Service. Explore the ship; it has many great areas. Very nice Solarium. Enjoy your cruise.
P.S. - this is why I had to wait until I had more time!