I should know the answer to this as a lifelong resident, but I'm drawing a near-total blank. I have friends (family with two teenagers) visiting from out of town for a few days, staying at a Metro-accessible hotel in town so they can see most of the major attractions without a car.
I'm playing tour guide one day with my car, so I want to give them some choices of the major attractions they probably wouldn't get to see without one. The only things that leap into my mind are Mt. Vernon and Udvar-Hazy. What am I overlooking?