The National Mall runs from the Lincoln Memorial through to the U.S. Capitol building. If you have never visited, it is absolutely worth a trip to see the monuments, Smithsonian museums, or the Capitol. The Mall spans many many blocks, so we recommend planning ahead to decide what parts you really want to see. rather than attempting to walk the entire thing.
This is one of our favorites museums. It is free to enter (like all of the Smithsonian museums!) but you do need to reserve a ticket in advance.
Pandas are back at the National Zoo! The zoo is also free, but requires a reserved entry pass. Because of the new pandas (Bao Li and Qing Bao), we expect the zoo to be crowded and recommend trying to reserve tickets in advance if you want to visit.
DC is blessed with a number of independent bookstores. Our favorites include Loyalty Bookstores, Lost City Books, Solid State Books, Bold Fork Books, and The Potter's House.
If you are looking for cheap(er) beer, greasy food, and good people watching, we love Wonderland Ballroom, Red Derby, Lyman's Tavern, and The Raven Grill (which only serves drinks, no food)
For a nicer cocktail or more extensive drink menu, we recommend Lost and Found, Church Key, and The Royal. Our favorite cocktail bar is All Souls (we got engaged here!), but have to warn you that the drinks are strong :)
Try the coffee and pastries at The Coffee Bar, Three Fifty Bakery and Coffee, and Soleluna