I think that a nice gift is a relevant history book. After my trip to DC i bought to myself a book about the women's suffrage movement after seeing a few exhibits marking 100 years to the constitution amendment
I have a feeling Nationals tickets will be a little more difficult to come by next season, so maybe give your loved ones some Redskins tickets, those tickets they practically can't give away :)
I know it would be rather subjective to each person like the best food but a video on what you consider the best souvenirs, memorabilia, or gift to buy in DC would be cool.