Well, I'm originally from Philadelphia, PA and the first thing that comes to mind when you say Philly is Cheesesteaks! Speaking of local fare, Philly has some of the best pizza, hoagies and soft pretzels that you'll find anywhere! (It's also the birthplace of Breyers Ice Cream!)
Of course, Philadelphia is home to our nations Liberty Bell, formerly on display at Independence Hall. In 1976 the bell was moved to Independence Mall and since 2003 it's housed at the new Liberty Bell Center. This city was once home to George Washington, John Adams, Ben Franklin and Betsy Ross. Historical documents like the Declaration of Independence and the Constitution were penned here and remain here. The Declaration was first read to the public on July 8, 1776 in Philadelphia. Atop City Hall in Center City (the world's largest occupied masonry building) there is a 27 ton statue of William Penn (the largest statue on any building in the world). This statue is so large that you can walk around the brim of the William Penn's hat. (virtual tour).
There's so much to see like the Art Museum and Fels Planetarium at the Franklin Insitute. Last but not least Philly is a sports mecca, home to the NFC Champion Eagles, Phillies, 76ers and the Flyers.