The C&O Canal Towpath is a pretty active place year round, especially in the summer. They currently have a program at the National Park entrance that offers anyone a free bike loaner for two hours with which to enjoy the trail.
You can step back into yesteryear and ride a canal passenger boat, hauled up canal by mules.
There's camping and fishing.
There's running and of course hiking as well. It's a well maintained national treasure that runs from the District all the way up into West Virginia.