The Washington DC area has a wide variety of bike trails. Also, some roads are closed on the weekends allowing bikers and joggers to really enjoy areas like Rock Creek park.
It is only a short trip west into Virginia, Maryland, or West Virginia to get to some great hiking trails with great views.
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It is also a short drive to some relaxing green areas. DC is a big city with numerous things to do but if you need some time away from town it is not that far away.