I would swing by NYC and hit the South Street seaport. There is a great exhibit called Bodies. It have everything from Mumification, to how the heart works, muscles and everything about the human body. Take 1/2 a day to get through the whole thing. I have a 15 year old boy and a 12 year old. Their granparents took them and they has such a great time they still talk about it months later. If you feel like taking in a show, we just saw Rock of Ages and its great becasue the music jump generations ans both my kids and wife and I loved it. With misic thats tough-thank God for Gituar hero and Glee! Then get tix to a Yankee or met game. The New Yankee Stadium is SICK-Go to the game 2 hours early and you can walk through monument part (it closes 1 hr prior to game time). There are serious restaurants in the part and the game is sure to please. City Field is also a new Stadium although since i am a Yankee fan, it just dosent feel the same to me. Could hit baseball Hall of Fame on the way to NY, its in Cooperstown. (Small Detour)
I went to school in Boston and love that city. I have taken the family there and Boston has so much to offer. On the way to Boston you have the Basketball Hall of fame, in Boston Fenway Park, Aquarium, Fanuel Hall, Duck Tour, Science Museam-Aviation Section is awesome-Wright Bros Through Space Shuttle.
I have two biys so most of our activites are geared to their interests, i hope you can use some of the above. In any case have a fun, safe trip and enjoy your grandaughter undivided attention. The fact that she wants you to drive her spreaks volumes about you and Her!