The shaded hiking-cycling path – part of a larger national historic park – runs alongside a waterway built in the mid-1800s to transport goods all the way to West Virginia. Step on at Jefferson St for a lovely green escape from the crowd. Note the canal and environs are being restored and enhanced over the next several years, so you might run into construction along the way; check the latest updates before setting off.
In its entirety, the gravel path runs for 185 miles from Georgetown to Cumberland, MD. Lots of cyclists do the 14-mile ride from Georgetown to Great Falls, MD. The tree-lined route goes over atmospheric wooden bridges and past waterwheels and old lock houses. It's mostly flat, punctuated by occasional small hills. The park's website and Bike Washington (http://bikewashington.org) have trail maps.