Beautiful. So many unique plants and fun landscaping.
This is a free museum/tour which is awesome but the norm in DC. It is a little far from the mall/main tourist locations but allows for a more real vision of all of DC, not just the touristy districts. The docents are knowledgable and fun and the views from the house are beautiful.
National harbor is a cute little spot to spend an afternoon with some shops, paddle boats and a Ferris wheel. The sculpture is pretty cool but is definitely a playground for kids on the beach.
I would pay for many of the experiences if I had a whole day to hang out in Williamsburg, if you only have part of a day it is free to walk around, see the buildings, eat lunch and shop in the old timey shops. It is a lot of walking and a really cool experience.
Yorktown is a nice little town to stop for coffee and walk around briefly. The museum is small with mostly a video about the revolution. The battlefields are ok if you are a history buff and you get to use your own car but I would skip if it is between this and williamsburg