I don't want to be a Debbie Downer, but if you are coming to DC this month, please make sure you pay attention to the federal government in the news and Congress's lack of passing a budget. There is a risk of a government shutdown, which means that all of the Smithsonian museums and the zoo and the parks will all be closed. Those are all gov't establishments. I wouldn't want you to make the trip and be able to see very little when you are in DC.
I'm praying those politicians figure it out and prevent a shutdown, but you never know. Please pay attention.
DC is awesome. I've never done the tours, but I have walked DC, and it is definitely very walkable, or just quick ride on the Metro (subway) to get to the further out sites (like Arlington National Cemetary, for example).