We stayed at the Embassy Suites convention center - that allowed us to walk to the Smithsonian - we took the Metro to the zoo and a bought day long tour bus tickets for the memorials. You will put a lot of miles on your feet.
Food in the museums is expensive…if you buy a Smithsonian membership - its discounted and the membership will pay for itself if you are eating lunch and buying souvenirs there. You can also eat at the food carts outside.
Contact your rep for tickets to the Bureau of Engraving and - if you want - the White House Tour. The White House Tour was OK, but a hassle. The Capitol tour is only worthwhile because its the capitol. Do stop in the Library of Congress building if just for the architecture (but the exhibits are interesting as well).
The zoo was ok - we did see the pandas which we’ve never seen anywhere else.
We did not get out of the city except to Udvar Hazy - and then only because we flew out of Dulles late, so we put our stuff in lockers at Udvar Hazy and saw the planes - amazing, but if you don’t have a car, it isn’t easy to get to. And into Virginia for Arlington.
My son and I did the Holocaust museum, my daughter and husband decided it was too depressing. Its VERY well done - but not a day brightener. We did the Spy Musuem, including the experience - expensive - it was the big splurge we made for that vacation - which all in all was a pretty inexpensive vacation, since everything else we did was free (except the tour bus tickets- which weren’t too expensive).
If you are looking for a relatively inexpensive place to eat, I’d recommend the food court in the lower level of Union Station. Unless things have changed drastically in the last 10 years it offers a fairly diverse selection of fast food type dining options - probably a better offering than a typical mall’s food court. It’s only a couple of blocks from the Capitol. In fact, Union Station might actually end up serving as your Metro hub depending on where you stay. Also, IIRC from when I lived on the Hill about 15 years ago, some of the metro stations in the area close one of their entrances/exits on weekends (hard to explain, but each station typically has two, which are a couple of blocks apart) which can be a bit of a nuisance if you are not expecting it.
I stayed in Virginia and took the train into DC. Hotels were more than half price, and train tickets were only $2-3 and took about 15 minutes to take me under the National mall.
We haven’t talked about food much in this thread. I’m not a huge fan of most of the offerings near the Mall (although the Museum of the American Indian has an interesting, if expensive cafeteria). There is tons of good food and around DC, but I’m not sure what’s worth the schlep for a tourist.
There are food cart clusters near some of the offices around the Mall for weekday lunhes. Plus some hidden food courts (L’Enfant Plaza comes to mind).
It’s so well hidden that I couldn’t find the L’Enfant Plaza food court! I think the directions we got were bad, and we ran across a McDonalds, which is what my daughter wanted, before I could sniff out anything better.