Explore the Black Heritage Trail
If you enjoy hiking, one of the best free things to do in Boston is to explore the Black Heritage Trail. There are 14 sites located around Beacon Hill that make up this walking tour, covering important parts of African-American history. Massachusetts was the first state to declare slavery illegal (in 1783) and you can learn a lot about the history of slavery and the African-American experience by taking this tour. Free maps are available at the Abiel Smith School if you want to do a self-guided tour, though there are several companies that also arrange guided tours.
Before the Civil War, the area was a key stop on the Underground Railroad, and the Black Heritage Trail travels through the Boston African American National Historic Site to 15 buildings that belonged to the city's African American community in the 19th century.
You'll get even more of a picture with a visit to the African Meeting House, the oldest (1806) church built by and for Black Americans, restored to its 1854 appearance to house the Museum of Afro-American History. More exhibits are in the 1834 Abiel Smith School, America's first public grammar school for African American children. The tour is self-guided, or you can join a National Park Service Ranger on a free guided tour from April through November.