44. Walk the Boston Freedom Trail
The Freedom Trail is 16 sites in downtown Boston stretching 2.5 miles between Boston Common and the USS Constitution.
The trail picks out many of the most significant sites in Colonial Revolutionary Boston as well as other landmarks (in the battle against slavery particularly) and is also intersected by the Black Heritage Trail.
You can easily spend a whole day on the trail because it traverses not only the historical parts of the city but attractions of Modern Boston too.