Hi there, DC area resident here. Have some ideas. First - what everyone else said about the weather- hot and humid. Drink a ton of water.
Note that many museums now require reservations/tickets, even if they are free - Holocaust Museum does for sure. I echo the suggestion that was made to do the Capital Tour, where you will also need reserved tickets. If you just want to watch the excellent film and have a short guided tour of the main areas, you don't need your Congressperson, but if you would like a more extensive tour, absolutely contact them. It is never too soon to make all of these reservations online, as soon as you have your dates. The Supreme Court is close to the Capital and you can have a look at the main chambers. It's kind of awe-inspiring.
Personally, I would skip Air & Space. It's a madhouse, and has become really commercialized. With so much else to do and see, unless you have young children, it would be low on my list. They've moved a lot of the exhibits out to the A&S extension museum near Dulles Airport, also. I was also really disappointed by the Museum of American History last year. I hadn't been for many years, and took a 14 year old visitor there. He was bored, and spent $10 to do some ride near the cafeteria which was his highlight. It was a total hodge podge of stuff, and a big area was being renovated so some of the most iconic things, like the ruby slippers and Michael Jackson's glove were not accessible. It's more fun for adults because there are things that make us nostalgic.
When you go to Ford's Theater, go across the street to the house where Lincoln died (need reservation!) It is part of the National Park Service. The little museum in the basement of Ford's is interesting.
The FDR memorial is sometimes missed because it's not on the Mall. It's very rich in detail. Great to grab a park ranger to talk about it all.
The Viet Nam Memorial is a must-see. It moves me every time, especially the mementos left there. Make sure to see the Nurses' Memorial, very close by. Korean Memorial is also really haunting. All are near the Lincoln Monument. These will all be beautiful at night.
Arlington National Cemetery is important, and the changing of the guard at the Tomb of the Unknown is worth seeing. I am not sure if you can still tour Robert E Lee's mansion which is at the top of the cemetery. I liked it several years ago because it was not all restored perfectly (they were asking for donations for restoration) and gave a good sense of the original time. ANC requires a good bit of walking, and is hilly though, so perhaps this is better for a spring or fall visit.
The Newseum has always been my one of my kids' favorite places, from age 12 on. It's not a Smithsonian Museum, so there is an entry fee, but it is a fantastic collection and a great place to cool off after lots of walking. And what is more timely than the NEWS these days? Close to the Newseum is the Navy Memorial and there is a small museum on the plaza dedicated to Navy and Coast Guard that is easily missed. DH and DS might like it - it's a quick visit.
The Spy Museum is fun. Haven't been there in a long time. It's close to Ford's.
Finally, I took the 14 year old on a Saturday morning bike tour of the major monuments. It was more fun and less stressful than I expected, and biking is cooler than walking. PM if you are interested and I'll dig up the company I used. It's a good thing to do on your first day to give you the lay of the land. I was the only local in the group.
DC is awesome, so glad you are coming, even if it has to be postponed for now.
