For history, check out the Liberty Bell and Independence Hall, where the Declaration of Independence was signed. Entrance to the entire complex is free of charge. Tours of Independence Hall are given every 30 minutes and require tickets (also free), but they must be obtained from a nearby location prior to the tour.
For nightlife, try either downtown Philly (on and around Sansom Street) or Fishtown. Each area has its own vibe, and both are fun!
City Hall and Rittenhouse Square are nice areas to check out too.
The Passyunk Square area is quite unique and has some cool artwork. There's a cozy cafe called Gleaner's that has an amazing homey atmosphere, great drinks, and insanely sweet staff!
Don't miss out on Wiz Kid either - their vegan rendition of a Philly Cheese Steak is out of this world!!