Philadelphia

East coast (explore a bit of the east coast first like New England regions, New York and maybe Philadelphia.

3 nights is fine in DC and Philly, you will have plenty of time.

And I hear Pat’s King of Steaks has the best cheesesteak sandwiches in Philadelphia.

Philly is closer so I'd vote for that.

Philly: 2 hours train ride Also, upstate New York has beautiful nature you can visit via the Metro North.

There’s tons of great food and culture (and public transit) in both Boston and DC, and those are both significantly closer to NYC. I’d plan a future trip to the Midwest where you can really see Chicago for the awesome place it is, and you can also check out Milwaukee or St Louis on the same trip, or other cities that are in that region.

Philly is closest, Boston also pretty close (relatively) and each has trains that are pretty easy. Depends on what you’re into— if you want to see American Revolution history type stuff, Boston is especially great for that (and you can also check out places in MA like Plymouth and Salem for 17th century history, though both are also very touristy). If you’re a foodie, Boston is great for seafood and chowder and stuff, Philly will have your cheesesteaks etc.

Other options would be Chicago, or a week visiting the Utah NPs, or tacking on to your NYC trip with visits to Washington DC, Philly, or Boston.

Philly is worth while if you haven’t been in a while

There's some underrated gems in the east coast such as Pittsburgh, Philadelphia, and Baltimore.