I am not sure when you plan to visit, but be prepared for CROWDs... especially at the popular "must see" items within the museums. For example, it took forever to get close to see the Hope Diamond, but there were lots of beautiful jewels (even more beautiful than the Hope in my opinion) with almost no one looking at them. I was there in early June, and the hotel was FULL of buses of school kids. Use of the pool in the hotel was almost impossible because of all the kids.
I do agree with using the METRO... where I live we don't have any type subway, but it didn't take but a little bit to get the METRO figured out.
I could spend several weeks visiting. I hit the high spots in several afternoons/evenings and then 2 long days... we only made the monuments on the mall, Arlington Cemetary and 2 of the Smithsonian Museums. Another thing to consider is the Sept 11 Memorial at the Pentagon. It is a really nice memorial and makes you realize how close we all came that day. This trip was primarily for work, with a few days of vacation tacked on the end. I stayed in hotel this trip in Arlington looking out over the cemetary and the city.