Bar Harbor is a charming town with shops and restaurants. There is not much to do in the town itself and an excursion into Arcadia National Park may be a good idea. Be prepared to tender as the ships cannot dock.
Portland is a great city and you can walk to some interesting places from the pier. The other option is to take an excursion to Freeport, home to the L.L. Bean flagship store. If you are a shopper, Freeport has been turned into an outlet mall thanks to Bean's presence.
If yuo don't want to be among the hordes going to Peggy's Cove, you can have a good time in Halifax. Along the harbor are shops, museums and restaurants. In Halifax you can visit the Citadel, an historic fort, and the cemetery where several hundred victims of the sinking of the Titanic are buried.
You would need several days and more time than is usually allowed in port to experience Boston fully. The ships dock downtown and within easy distance of the center of the City. Faneuil Hall, Quincy Market and the North End in particular are close by and filled with tourists and locals. If you have time wonder farther afield you can walk the Freediom Trail, visit Copley Square or venture to Cambridge to see Harvard Squarre.