I was born in Philly.

Vine Street (I think that's the name!) is home to the Franklin Institute, the Academy of Natural Sciences, the Courthouse and the Main Library.
I LOVE the Franklin Institute-- it's an interactive children's science museum, but there is plenty for grown-ups to enjoy. There is a giant replica of a human heart you can walk through. When I was a kid, I was terrified, but I always made my father pick me up and carry me through it. Later, visiting Philly as an adult, I have no idea why I was so scared about a plaster tunnel with lights and a "bump bump" heart beat sound. LOL. You can climb all over it, too!
The Academy has a great dinosaur exhibit and used to have (might still have!) an animatronic dinosaur in the downstairs commissary.
Definitely check out the Liberty Bell, Independence Hall, Penn's Landing (docks/yachts), Ben Franklin's grave, eat cheese steaks (with onions and ketchup!) and soft pretzels. And the Mummer Museum! The Mummers are an old Philly tradition-- they make incredible costumes and floats and parade through the city on New Year's Day. It's a great local tradition.
If there's time for a show, my family used to really enjoy the historic Walnut Street Theatre.
Lots of Irish pubs.
I think the mall down there is the Galleria. It's pretty large. Hope this helps!