My family and I did a similar stay about a year ago though it was a paid stay rather than an award. We stayed part of the time at the Crystal City Marriott and the rest at the Crystal Gateway Marriott. They are literally right across the street from each other. We chose those two because the weekend rates at the Gateway Marriott were lower and the Crystal City had lower rates during the week. As mentioned, they are perfect for metro access and very close to the airport (and unless you come very late, there is a complementary shuttle to the hotels.) If I remember correctly, we all received free breakfast even on the weekends at the Gateway Marriott.

I would recommend the Gateway Marriott as the better of the two. When we stayed there (two rooms on awards), they upgraded one of the rooms for us to a suite room. ^ You won't need to rent a car at all. Take the shuttle to the hotel when you get there, and use the metro for the rest of your stay.

Also, while I like the Marriott Suites that VA1379 mentions, I only stay there when I'm working in Herndon or Reston. You will definitely be a bit outside of DC if you stay there. If you are planning on spending most of your time inside of DC itself, I would suggest getting a hotel there rather than worrying about getting a car, traffic, etc. Actually, if you really want to stay that close to IAD, I would suggest the IAD Marriott. It is actually on the airport grounds and a fairly nice FS Marriott. The benefit of staying there is that since you are on the airport grounds, when you are going/coming to DC, you can use the "inside" lanes of the toll road. Much less traffic and you don't have to pay the toll either. ^