The River Line passes through several historic towns along the Delaware River as well as terminating (ending) at the Camden Aquarium and battleship. You can get off at these locations and explore.
Bordentown: Historic homes and restaurant district. Historic homes to Thomas Payne, wrote "Common Sense" which convinced many colonists that the cause of the American Revolution was righteous, Sarah Barton, well known Civil War nurse, educator, and figure in woman's history, and Napoleon's brother (Home no longer exists.)
Burlington: Probably the most under appreciated historic district in NJ. Founded in 1687, Burlington has a beautiful historic district with over 40 historic sites. You can learn about the Underground Railroad with several sites in the city. Civil War commanding general and later U.S. President, Ulysses Grant lived here. There's a Quaker Meeting House, one room school house, prison museum, home built in 1685, folklore includes a witch trial and pirate buried treasure, home to James Cooper (Famous author), and views of the Delaware River.
Riverton: River town with many Victorian Mansion. Nice coffee/bakery house, tea house, and ice cream stop.
Camden waterfront: The aquarium houses many species of fish and other aquatic animals such as hypos.
Nearby is a retired battleship to tour. (Fee) Nice view of Philadelphia and Ben Franklin Bridge from a promenade on the Delaware River.
In the summer, there are boats to Philadelphia.