In the old days, the trail beside the C & O Canal was used for mules to tow small barges laden with freight. Today, it's one of the finest bike trails in the country, encompassing 184.5 miles of nature, history and local curiosities between Georgetown and Cumberland, MD. Following the course of the Potomac River, the route begins at the canal's notorious Watergate and passes such notable sites as Harper's Ferry, a training facility for whitewater rafting, and a colorful little pub called Dollar Bill's, where patrons attach dollars to the ceiling. There are also numerous sites to pitch your tent for the night.
If you go:
The most reasonable long-term parking on the DC end of the trail is at Reagan Airport. In Cumberland, this trail connects with the Great Allegheny Passage, which will take you another 120 miles to the outskirts of Pittsburgh.
Links:

1057 Thomas Jefferson St. NW
Washington, D.C. 20007 USA
Mailing address:
1850 Dual Highway
Suite 100
Hagerstown, Maryland, 21740 USA
Phone Numbers:
Office
+1 (301) 739-4200
+1 (301) 739-4200