We found a wonderful place to camp in a National Park (Greenbelt) not too far from their house. It was hard to believe that a National Park in a major metroplitan area could feel so much like the country. The woods were full of deer and other mammals. We took the Metro into the city for every visit from a stop not too far from the campground.
We visited the C and O Canal (http://www.nps.gov/choh/)on the Potomac River west of the city. We had a wonderful day riding our bikes along the path on the side of the historic canal. We were amazed at the falls along this river. Despite the crowded path that was sometimes hard to ride in and out of the all the people, we had a great time and enjoyed some spectacular scenery.
On another day we visited a natural area along a river that lead to the Chesapeake Bay. We had hoped to paddle in the marsh but low tide and cold temperatures prevented it. As we were walking the trails in the area, we met a guy who had gone to school at LSU to get his masters in wildlife biology. We enjoyed talking with him about many different subjects.