I would NOT make the trip to Philly or NYC. It's a huge hassle with either the train or, God Forbid--driving. Worst traffic (train and auto) in the U.S. on 95. The train is OK, IF there aren't any track problems--which aren't as common as people want you to believe, but are an issue) If you want a short couple of days, trekking up that congested mess isn't worth the time. Look east from D.C. to the Eastern Shore of the Chesapeake Bay, to Cambridge, St Michaels, or even Annapolis, MD. You can probably find small buses running to each. Nearby MD and Delaware Atlantic beaches are also not far away. Rehoboth Beach in Delaware is one of the more popular "DC getaway" beaches and near Ocean City Md.
You can get a bus to either place. All are within a relatively easy trip form of D.C. and are vastly more easily accessible than NYC or Philly.
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Also, as said, the Blue Ridge Mountains to the West are accessible and (IMO) have some of the most beautiful places in the U.S. The Shenandoah Valley of Va. is a particular gem, and a three hour drive from D.C. on mostly open highways (once you get out of the congestion of the D.C. metro area). Charlottesville, Orange, Harrisonburg, and Staunton, VA are all in the Valley or very near it. Wilderness areas are nearby as well.