Recommended Car Trip routes

The first leg of that trip (Boston to Niagara Falls) is the longest.  So I would suggest a few stops on the way there - perhaps Old Sturbridge Village (Massachusetts) (an hour west of Boston -it is an outdoor museum) and somewhere in the Finger Lakes region which would be another 5 hours.  From there it is only about 3 more hours to Niagra Falls. Then on the way east, drive along the shore of Lake Ontario to Rochester (New York) and then go to Burlington (Vermont)  Drive down US7 to the Green Mountain  If you go this way it avoids 'backtracking' but it adds several hours to the trip, so you will probably want to break it up into three days, perhaps staying up by Rochester one night and Burlington a second night.
From here it is about 4 hours to Mount Washington and Bar Harbor is about another 4 hours.  On the way back to Boston, you might want to stop at Strawbery Banke Museum (which is an outdoor living history museum) or you might want to visit Salem (Massachusetts)before you go back to Boston.