I arrived in DC at the Reagan airport, and was able to figure out how to get to Ballston via subway (it is about ten stops including a metro change). The subway stop is about five blocks from the hotel (which is in downtown Arlington I presume). My conference was three days, and each evening we went out for dinner at a restaurant within walking distance. I checked out the mall (a block from the metro station). If you don't feel like walking, the complimentary Holiday Inn shuttle will drop you off at nearby places (but not to the airport which is perhaps ten miles away).
I went on a long (3 mile) walk one evening. The area seems perfectly safe. Our conference price for the room was $180 per night (which seems like a lot in a small city), but I don't know, maybe that's normal.
