As someone who lives 45 minutes north of Philadelphia, welcome!
It depends on how history-oriented you are... it is something, to see Independence Mall where the country was founded, that's something. The Liberty Bell is an easy one. Right next to it is the Constitution Center where they have regular shows about the C.
Food oriented? Go to
this website and have a cheesesteak from any of the top 20 places that have more local % than tourist %.
You can get an authentic Pennsylvania-oriented breakfast or lunch from the Down Home Diner in the Reading Terminal. Although there are many places to get a meal there. If you want the most amazing meat sammich of your life, go to Hershels East Side Deli there. Pastrami, corned beef, turkey, and I think brisket? Hand carved before your eyes, and just excellent.
There are parking lots downtown, pretty expensive. Getting street parking is unlikely downtown, possible in other locations.
Across the river is the
Aquarium which both young tykes and parents enjoy.