Washington DC has plenty to see. The Smithsonian Institute alone has 19 museums. The International Spy Museum was interesting. Ford's Theater was being renovated when we were there a few years ago but it might be open now. The Library of Congress was wonderful. There is a textile museum (can't remember the name) on S Street that was interesting. A little way away in Quantico, Virginia there is the Marine Corps Museum (we haven't been there but I heard it was good). When we were in DC a few years ago, the Metro didn't run around the clock. I wouldn't drive in DC if I were you. They drive like maniacs and I was glad that DH did any of the driving.
I would avoid the Grayhound idea. The last time I was on one, it was disgusting. Many of the bus stops are in no so nice areas of town.