Head to the mall and you can take in as much as you can stand or possibly see in a day, see Lincoln at one end, the Capital and the other and natural science, art, etc, in between. You'll hardly beat a bicycle you can carry the little one on if the weather is nice. People watching on the mall is a interesting adventure in and of itself. My favorite restaurant in the DC area is Clyde's at Tyson's Corner, great menu and request reservations/seating in the waterfowl room. If you have time to get away from the metro area, go to the Ward Brothers Wildfowl Museum in Salisbury, MD.