OK, I have a few ideas for off-the-beaten-path stuff, but you should know that without a car, it is harder to get to some. For example, Mount Vernon is a short dive, but it is a drive (unless you take an organized tour). It is a gorgeous place to visit, especially in late summer or fall. It sits on a promontory over the Potomac so the views are spectacular. It is also someplace where there is nothing between you and the full-on historical experience. You walk the same floorboards George and Martha walked and stand in the room where he died. If you are a history buff, it is a trip worth making.
Even if you don’t make it to Mt Vernon, I highly recommend Old Town Alexandria. It is an easy-to-walk area of beautiful late 18th/19th century homes. MUCH less crowded than Georgetown. Lots of restaurants (mostly middling – but Pizzeria Paradiso) and shops, but there is also the Torpedo Factory, which houses studios for a variety of artists (many of extremely high quality) as well as a gallery where they sell some very nice (non-kitch) stuff. From very small $25 wood block prints to large oils and everything in between. You can get to Old Town by water taxi from Georgetown and the Navy Yard (the latter has a metro stop), which would be fun.
Georgetown can also be fun to walk unless crowded sidewalks send you over the edge.
The Smithsonian museums of course are not really off anyone’s path, but if you want some ideas there let me know. The Air & Space Annex at Dulles is off the beaten path but unreachable without a car (I guess there are buses from the hotels but yuck)
The other Smithsonian that is (a) off the Mall and (b) less crowded is the combination Museum of American Art and Portrait Gallery (Steven Colbert!). It is in the middle of Penn Quarter, surrounded by really good restaurants.
Oh, and one of my favorite rooms in DC is the National Building Museum. At one time it was one of the largest office buildings in the world. It was built in ~1870 to house the Pension Bureau for civil war veterans and to provide a grand space for public events. It has housed a presidential inaugural every four years since it was finished. It is a breathtaking piece of architecture which makes it a fitting place for the collections/rotating and traveling exhibits.
While on the Hill, you should definitely stop at the Library of Congress. Aside from the reading room, which is a spectacular space, you can see the Jefferson collection. After the Brits burned the Capital and destroyed the library, Jefferson sold some ~6,500 books to the nation. A second fire ended up destroying about a two-thirds of those books, but the remainder are on display, along with same-edition replacements hunted down and purchased by the library in recent years.
Down the block is the Supreme Court, and if you are there on a Monday or Tuesday in October, you really should take in an argument. If the cases being heard aren’t abortion or similarly newsworthy, I am guessing the line wouldn’t be too long. (Supreme Court bar = no waiting! so I don’t have fist hand experience). An argument is an hour long, so it won’t eat too much of your day. If you go, stop by SCOTUS blog first and read a little about the case so you can follow.
Outdoor spots on metro lines are harder to find. The Zoo, of course, with the pandas. Horseback riding in Rock Creek is fantastic but you have to book soon (and take a cab to get there). The National Botanical Gardens is nature-y but indoors – a hot house at the western base of Capital Hill, and can be a quiet and nice place to recharge after slogging through the Cap or the museums. The National Arboretum is nice, I hear, if you like that nature stuff.
Check who is playing at 930 Club and Birchmere; both are regarded as among the best live clubs anywhere. 930 was a birthplace of punk in the US and Birchmere is mostly roots rock, folk, country. Blues Alley is a fantastic spot for jazz; food is only edible but the best acts in the country play there. Tiny room. Bohemian Taverns is a renovated spot on U Street; everyone who was anyone from the 20s to the 50s in African American music played there. I havent been since it reopened
Lots of theater if you are interested; let me know.
I’d do the White House tour if you are interested in history. It is a bit of a pain to arrange, and the tour is limited, but hey – it’s the house where every president since J Adam has lived. Walking where Lincoln and Fillmore and Polk walked. And while you are there, spend a few minutes walking around Lafayette Square. Lots of history there; read up.
Decidedly not off the path but worth considering: the Washington Monument has reopened after the earthquake. Not for the claustrophobic
I’d also visit Ford’s Theater and the Peterson House. Because.
The farmer’s market in Dupont Circle every Saturday is terrific and a great excuse for people watching.
OK, that’s enough for now. Let me know if you have special interests and I can refine.
