First visit to the US

[Szykneij]

Dave --

What month are you planning to visit and are you a fan of the heat?

We took a similar vacation a couple of years ago in August. We had a great time in New York City but arrived in Washington, D.C. at the start of a heatwave. The temperature hovered around 105 degrees F (about 40 degrees C) all week.

It was the most oppressive heat I ever experienced until I visited New Orleans in July.

[The Six]

Just stay away from Boston and you'll be fine. 

[DaveF]

Perhaps.

Ah, c'est le Périphérique!  OK, we will éviter comme la peste.  Merci.  London's the same - best way is straight through the middle (avoiding the congestion charge zones) rather than round either of the ring-roads (M25 or North/South Circular).

What month are you planning to visit and are you a fan of the heat?

School term finishes on 6th July (bah! would have liked to be in NYC 2 days previously for what is alleged to be a whole lotta fun), so as early as possible after that for 2 weeks.  A moderate fan of heat (it's so long since we had any here that I struggle to remember), but our non-package-deal flexibility means that if things get too oppressive we can head for the hills.

Just stay away from Boston and you'll be fine. 

Ouch!  Because of the heat or because of... Boston?

[(poco) Sforzando]

It's really unfortunate that one snarky, unexplained comment can lead you to ignore as enjoyable a tourist location as Boston. It's a great town to visit, with a major art museum that bears comparison to New York's and numerous interesting historical sites. As for the heat, 2012 was a very hot summer, but unless this trend continues, summers in northeast USA are warm and humid but not unduly oppressive.

Too bad you'll miss the July 4 celebration, but truth to tell, you can see it on TV and once you've seen one firework, you've seen them all.

Since you seem to want to stay focused on NYC and DC, here are some thoughts of mine on places to see.

New York: For art museums, the Met and MoMA for sure, the Guggenheim and maybe the Frick and JP Morgan Library if you have time. The Cloisters is the separate medieval branch of the Met way uptown, but worth a detour. Go up to the top of the Empire State Building for the views, and all the way downtown for the Statue of Liberty. Lunch in Chinatown followed by a walk up to Little Italy for the pastries at Ferrara's. Other small neighborhoods downtown worth seeing include Greenwich Village and the Lower East Side, and Chelsea if you're interested in the art gallery scene. Definitely tour United Nations headquarters. Many people are afraid to see Harlem because that's where (shhh!) the blacks live, but you can get a bus tour, which is best on Sundays because you can visit one of the churches to hear the gospel singing. For music, see what's doing at Lincoln Center and Carnegie Hall. If you want to catch a hot Broadway ticket like the Book of Mormon, book early from home (the language might be too adult for a little kid, but use your judgment). In summer you might want to catch a play at Shakespeare in the Park. The outer boroughs are less interesting than Manhattan, but your kid and the kid in you will enjoy a trip to the Bronx Zoo, and the Brooklyn Museum is also very fine.

Washington, DC: On the National Mall, see the NASA Museum, the Museum of American History, the Holocaust Museum, and the National Gallery (both west and east wings). The best small art museum (impressionist/modern) is the Phillips Collection near Dupont Circle. The Washington Monument may still be closed from the earthquake a couple of years ago; I wouldn't waste time seeing the Lincoln or Jefferson, but the two monuments on the Mall not to miss are the Vietnam Veterans' and the recent WW2. If the FBI Building reopens tours, don't miss it. You will probably need to contact your embassy to see if you can get tours of the White House and the Capitol; I would not miss either. Arlington Cemetery is not worth the bother; the Library of Congress definitely is. See what's playing at the Kennedy Center for music. Mount Vernon is worth a day trip, same for Williamsburg.

As for driving, you can find congestion in any major US city any time, so you should feel right at home. All the same, I honestly don't see the need to drive at all; you can easily get around either city by subway or bus, and you can avoid the trouble and liability of a car by using Amtrak. And of course we drive on the right side of the road, which you may or may not be used to.

[Karl Henning]

Just stay away from Boston and you'll be fine. 

Oh, gosh.

[(poco) Sforzando]

Oh, gosh.

Gosh indeed.

Edited to add: if you want to see just one natural history museum, I would choose the American Museum in NY over the one in Washington. Your kid will love the dinosaurs. If you want to see a major library, I would see the Library of Congress over the 42nd Street main branch in NY.

For the Museum of Modern Art in NY, the high point is the 5th floor if time is short.