Any reason why you're not heading too far south? There's great stuff to see in North Carolina, Tennessee and Georgia.
The Appalachian Trail is also beautiful if you want to see some more natural beauty.
Yep, AT is brilliant.
She had to have her way eventually after two years of your choice of driving holidays!
Slight aside, I looked into doing this sort of trip and was struggling with cars, really wanted a Mustang after having one in Vegas but they seem much harder to come by on the East coast, there were other options but not knowing a huge amount about American cars there is a serious risk of a dud. There is after all no point in going to the states and renting a focus.
Isn't the whole idea of going to the US to see the sites and culture?
Personally you need around 4 days in Boston to see it reasonably properly. I was there for 3 and only saw most of it at a rush. The Freedom trail and USS Constitution for a bit of history, Whale watching tour for a bit of nature (about 4 hours and only $30, saw about half a dozen Humpbacks very close up) and then half a day round Harvard and it's museums etc. I managed to do the Freedom trail, constitution and Harvard in a day but it was pretty knackering and rushed (for example I literally rushed round the campus and didn't go inside anything). Then there's the other areas round it, which I guess you could stay in.
If you're interested in seeing the US, not just the cities then head up to Maine, proper "back country" with moose potentially seen from the road/on the road and loads of lakes and wilderness to play in, it's only a 4ish hour drive to Bangor in central Maine.