First you might get your reservations in order ASAP. You'll be travelling the week right before the 4th of July and you may already have a difficult time getting reservations in many places.
I could easily skip NYC and all the traffic, parking hassles, etc.
Your girls might enjoy Mystic Seaport Museum along the Connecticut coast.
If you want to do Cape Cod, consider camping somewhere around Sandwich, MA, (maybe Scussett Beach?) and taking a day to drive out and back. Maybe a second day at Newport RI. Interesting town.Tour the mansions or do the Cliff Walk. We even took in Boston from the Sandwich area by taking the train into town. A much more civilized way to do it.
Bikes are really fun to have at Acadia NP. There are miles of old carriage roads open to bicycles and walking. You can also rent bicycles for the day in Bar Harbor. Gets one away from the traffic noise and by lakes not accesible by car. Definitely something we'll do again when we go back. The two NPS CG's on the island are both nice, but no hookups. Some sites are close enough to the water to hear the surf at night.