
In the 18th century, Philadelphia was an important port, with ships coming and going, carrying cargoes from around the world. It was also a center of shipbuilding, and founding of the US Navy. Independence Seaport Museum is a great way for kids to experience Philadelphia's maritime history.
Outside the museum, walk out to explore the Olympia, a cruiser from the 1890's used in the Spanish American War, and submarine Becuna, a typical WWII submarine (included in museum admission). Also check out the impressive sailing ship Moshulu, which has a restaurant.
The Tidal Delaware River Trail runs along the river, and kids can get out and run around - it's fun to just watch the boats on the Delaware River.
Spruce Street Harbor Park - Small kiosks serve local Philly food outdoors - pizza, doughnuts, Franklin Fountain ice cream, also live music, free shows on Thursdays all summer. May to October.
Riverlink ferry - May to September, take the ferry across the river to Camden New Jersey. Stroll the waterfront park (bring a picnic), visit Adventure Aquarium and battleship New Jersey.