Cape Cod Rail Trail
Harwich rests near the beginning of the 25-mile bike path that runs along the former railroad. Lots of nice paths and lakes nearby. Bring a bike. Though walking could be pleasant too, and make it easier to leave the trail when you want to check out a path.
First expored in 1619 (by Thomas Dermer together with Tisquantum, a captured Pawtuxet Indian), and subsequently seen by white men only in 1626 when a Scottish ship sank in Pleasant Bay, Harwich didn't really get much attention until just before March 2, 1640, when a charter was granted for a new Colony there. (Thanks, Harwich Historical Society)
Much of what is worth seeing here, such as the building pictured at left, was built by the grandchildren and great-grandchildren of the original colonists.
Harwich is also notable for the fact that the Cape Cod Rail Trail, a bike path that runs the length of Cape Cod along the former train tracks, passes through (actually, I suppose nearly all the Cape Cod towns are "notable" for that). It also has a lot of nice lakes, though access to them can be non-trivial.