We used Toumobile once. ....
I love Ford's Theatre. And agree about the old post office. We found ourselves in Natural History over and over again, which surprised me (I like American best, and I thought my husband would drive more time at Air and Space, but my daughter took the lead, and for her it was Natural History). I'd skip the Capitol, although the Library of Congress is a STUNNING building (and usually has interesting exhibits, including a Gutenberg Bible). Udvar Hazy is way out by Dulles, but is a great place to visit.
Get tickets in advance for the Holocaust museum online. $1 per ticket will get you an entry time. If you don't, you are at the mercy of "when the next tickets are available day of" - sometimes that is "not at all."
I did a moonlight tour in high school and did like that better....more limited time. I wouldn't walk the monuments, while it is "walkable" it is a really LONG walk. We walked from Jefferson to Roosevelt and the kids were dragging (and they are Disney vets).
I'm sorry to hear the Tourmobile went out of business. We chose it because it did Arlington, and I wanted my kids to see that...the changing of the guards in particular. And the Lee house, because we didn't get out to Mount Vernon, so the Lee house was the only chance they had to see that sort of thing. And because its had such a long history. The fact that our much indebted government was getting a small portion of the take helped too.