Rosslyn is the most northern part of Virginia right outside Georgetown, if you wanted you could walk to the National Mall from there although a bit of a hike it's very doable and you can walk over the Roosevelt Bridge and the Francis Scott Key Bridge. There are plenty of rent-a-bike or scooters available in Rosslyn and your car is safe in those parking garages around there where a variety of government contractors and agency buildings there are like Deloitte and Accenture.

There's also a Metro stop that can connect you from Dulles / Reagan airports on the Orange, Silver, and Blue line to get to Rosslyn. Although I don't know if the Dulles airport stop is up and running yet, it looks close to, there are other Silver stops nearby the airport you could catch a taxi to.