Just read through the rest of the thread.
We did a trip a few years ago that involved spending a 24h in Philadelphia - BIL lives there and he was our tourist guide. We did the things I mentioned earlier.
The next day, we drove up to NYC, which was about an hour and a half away. We had a place booked for the night, and the main challenge was finding secure parking.
We spent basically 36 hours in NYC, and then headed north towards Boston, as we were having a family reunion on Cape Cod. We spent the night at Mystic, CT, stopping at a HoJo's or something similar.
DH took the boys to an aquarium and I took the girls to Mystic village (a shopping plaza). Then we got in the car and stopped in Newport RI for lunch (very busy - we ended up eating at Subway, which was not the experience I was after). We arrived in Falmouth, Cape Cod around 4pm and stayed a week. We were in a motel, where weekly lets was the norm, that had a pool and beach/dock. We didn't really feel there was an awful lot to do at that part of the Cape. I think that kind of holiday would be disappointing if you were coming from the UK.
One of the things to consider is where Americans go on holiday. Most of my acquaintances drive to the Outer Banks of North Carolina (400 miles from Philly), and rent beach houses.