First US Road Trip - 3 Weeks NYC Loop - Looking for advice on what to fit in

[NewEyes]

Hi, This will be our first trip to the US from the UK and will be flying in and out of New York, staying 4 days and then picking up a motorhome/RV. Me & husband and possibly 19 yr old in either May or September. We have been camping in UK for years & undertaken some European road trips in the past.

Current thoughts are:

New York City - 4 days to see the sights

Washington DC - plan to stay in RV Camp and take train into Washington and see the sights,

then move up to Niagara Falls - It looks advisable to break this drive somewhere but not sure where best

Go somewhere in Finger Lakes - Watkins Glen

Adirondacks - Go up to Lake Placid? or is that too far?

Boston

Cape Cod - Whale Watching Trip

See some Lighthouses on trip back to New York

Any advise welcomed - best places to see, we'd like to see & experience a variety of things along the way, this loop seemed to give some city stops, some lakes, mountains and touristy elements. We don't want to be driving all day every day and want to have some moseying time along the way. We love the water, the more places you can swim the better. When we go to Greek Islands we have been known to go down every road on the map to see the beach at the end, but the distances are a bit more daunting in the US and we need some help in our planning.

thank you for your time