This is a great itinerary -- and one we've done a few times. We also live in the area.
MV - My best advice here is to rent a bike and do your own tour. There are many maps available, and bike rentals are in abundance near the tender dock. Just watch the time as it is a tender port.
Boston - The Freedom Trail is a must if you are into American History. There is no charge -- just follow the red line along the sidewalk. Maps are also available online. The cruise lines, along with the port of Boston, generally run a shuttle (for a fee) to the Faneuil Hall area, which is on the Freedom Trail. As the Trail is a large circuit, it doesn't matter where you begin. Another choice would be a Boston Duck Tour, in which you are driven through the city in an amphibious vehicle. Part of the tour is on the Charles River itself... very pretty and relaxing.
Bar Harbor - A great town for walking. And,the main attraction is Acadia National Park -- well worth the visit. You don't need a ship tour for this, but again, watch the clock as this is a tender port. When you are done with the Park, take a walk through town and stop just about anywhere for a lobster lunch. Trust me - you will never enjoy cruise ship "lobster" after this.
Newport - The mansions are the main draw. My favorite thing to do here -- as a sea-lover myself -- is to walk along the area behind the mansions where the Atlantic meets the rocky coast, an area known as the Cliff Walk. You won't have a lot of time to visit more than a couple of mansions. My favorites are the Astor House and the Breakers. If you choose to stay in town, there is the tennis hall of fame and the oldest surviving synagogue in the mainland U.S.
Halifax - This is my favorite port. (In fact, we were there again just a couple of weeks ago.) Peggy's Cove is very pretty, but if you are already doing the Cliff Walk in Newport I wouldn't bother. Instead, I recommend the Maritime Museum of the Atlantic and the Pier 21 Immigration Museum, both located on a boardwalk along the pier. (In fact, the ship will likely dock at Pier 21.) You can also visit the Citadel fortress -- again, for a bit of history.
Obviously, I can go on and on about these ports. If anything in particular appeals to you, please let me know. Also let me know if there are any other activities you'd enjoy, such as beaches or shopping.