The National Constitution Center chronicles all things related to the U.S. Constitution, and its interactive exhibits make that era come alive. These days, the center is the diamond in the center of Philadelphia's most important "birth of America" sites.
Nearby, Independence National Historical Park is home to Independence Hall, considered the birthplace of America. Tours are free, but for most of the year you'll need a timed ticket to enter. You can reserve in advance online or by phone (for a small fee), or show up the morning you want to visit. Arrive early during the busy summer months.
The Liberty Bell is another symbolic Philadelphia classic, full of legends relating to America's early days (in fact, the bell was hidden away in nearby Allentown to keep it from falling into British hands during the Revolutionary War).
Beyond the usual Independence Mall attractions, the Old City neighborhood features a number of other sights to see, including the Betsy Ross House, Christ Church and the United States Mint.
Art lovers shouldn't miss the Philadelphia Museum of Art, which houses the city's most stellar collections and visiting exhibits. Be sure to stop in the lovely medieval cloister and the tranquil Japanese teahouse. It's also got the famous set of steps featured in Sylvester Stallone's "Rocky."
Just down the Benjamin Franklin Parkway is the Rodin Museum, a smaller museum that showcases the largest collection of the French sculptor's work outside of Paris. Also nearby is the Barnes Foundation, an incredible collection featuring Post-Impressionist and early modern paintings from artists such as Picasso, Cezanne, Renoir and Matisse.
You'll find two other popular museums right on the Parkway. Our favorite thing to do at the Franklin Institute is to walk through the bigger-than-life human heart (trust us, you've got to be there) -- but it offers a wealth of other creative and informative exhibits on science. The Academy of Natural Sciences of Drexel University is beloved by families for its fun collections of dinosaur bones and other wonders of natural science.
A fun option for kids is the Philadelphia Zoo, America's first, which in addition to the usual animal suspects also has a fantastic "Zooballoon." This hot-air balloon rises to 400 feet and offers wonderful views over the city.
Adjacent to the gorgeously picturesque streets and houses of historic Society Hill (home to Philadelphia's wealthy in the 18th century) is Penn's Landing, Philadelphia's waterfront on the Delaware River. The highlight here is the Independence Seaport Museum, displaying numerous maritime artifacts.
In Center City, don't miss a stroll through Chinatown (centered at 10th and Race Streets) or the Pennsylvania Academy of the Fine Arts, which showcases a fabulous collection of American masterpieces within a landmark Victorian building.
Enjoy one of Philadelphia's loveliest neighborhoods: Rittenhouse Row. Centered around an urban park, Rittenhouse Square, the neighborhood is full of Philadelphia's hippest, trendiest and chic-est restaurants and boutiques (the park is plenty nice too). Aim for Walnut and Sansom and explore from there.
A trip to a prison may not top your must-do list on vacation, but the Eastern State Penitentiary is a surprisingly fascinating stop. You can wander the haunting, crumbling corridors that once housed Al Capone and numerous other criminals, and learn how revolutionary the "penitentiary" concept was at the time of the prison's founding in 1829.
The Mutter Museum may be Philadelphia's most unique attraction, exhibiting medical oddities such as a human skull collection and a tumor removed from the jaw of President Grover Cleveland.
Philadelphia's Magic Gardens are a beloved indoor and outdoor art installation stretching along half a block on South Street. Created by local resident Isaiah Zagar, the installation is made of mirrors, bicycle wheels, glass bottles and other found objects.
The Mural Arts Program runs various tours (foot, trolley, train) to explore the city's colorful public art.
Just across the Delaware is the city of Camden, New Jersey, which is experiencing a rebirth of its own. Major attractions include the Adventure Aquarium -- the most popular exhibit there is a 40-foot shark tunnel, where viewers are surrounded on all sides by a variety of circling sharks -- and the Battleship New Jersey, America's most decorated battleship. Families can also wander through the Camden Children's Garden. To get to Camden from Philadelphia, board the RiverLink Ferry at Penn's Landing.