GoPhila.com has customizable audio city tours that you can download for free. There are tours on themes such as history, food, and even one featuring the recommendations of locals. You can download a guide in its entirety, or pick and choose from the different tours and create your own.
This is a great resource to know about, not only because it's free and interesting but because instead of downloading an inflexible tour, you can line up the day that you want and then have historical tidbits, practical information, recommendations, and maps ready to go. Not bad, Philly.