New York to Boston

We are planning a 3 week summer vacation with 2 teenage daughters (17 and 14). We will be spending 5 or 6 days in New York, and then will pick up a car. We have a flight booked out of Boston, and are intending to spend 3 or 4 nights there before our flight home. For the intervening period however, we are unsure of which route to follow. As well as the city time we would also like if possible some beach/relaxation time.

Plans 1 involves a drive to Niagara but not sure if this would require an overnight stop en-route, as we don’t know what attractions, historical sites etc are on the route. We envisaged a day in Niagara to visit Niagara on the lake, caves, Maid of the Mist etc. After that, we are unsure whether to head into Canada for Toronto, Ottawa and Montreal, or to remain in the USA and head over towards the coast. How long should we allow for this drive back to the coast, and any suggestions as to a suitable route.

Plan 2 would involve remaining within the US and driving towards Boston with stops en-route. Our knowledge of this area is very sparse and so we are plucking places out of a hat when I mention Cape Code, Stowe, Salem, White Mountains, Green Mountains etc.