For anyone interested in history, engineering, architecture, or even the importance of clean water, the Fairmount Water Works is an informative (and free) attraction. Philadelphia was the first large American city to deem the delivery of safe water to its citizens a municipal responsibility, and in fact, the city's water department was the first in the US to supply an entire city with drinking water. Philly's Fairmount Park, one of the nation's largest urban park systems, was originally founded in order to protect Philadelphia's drinking water supply.
But back to the Water Works... Powered by the Schuylkill River, pumps raised water into reservoirs high atop a nearby hill, Faire Mount (which got its name from William Penn himself, and upon which the Philadelphia Museum of Art now sits). And from those reservoirs, the water was distributed to the city. The huge machines were concealed behind a riverfront complex of lovely neoclassical buildings designed by Frederic Graff and Frederic Graff, Jr.
Considered an engineering marvel in its day, the Water Works was an extremely popular domestic and international tourist destination in the 1800s.
Located just behind the west entrance of the Philadelphia Museum of Art, the Water Works is one of my favorite places in all of Philadelphia. Admire it from the outside, or head inside to check out the inner workings.