Here are all the areas in old city worth looking at - you can get a good number of these done if you get in early and don't mind spending the day
The Constitutional Walking Tour of Philadelphia
Getting to Reading Market will involve a long walk (on a hot day) or a quick "El" ride or a "Phlash" ride from the historic area...walking 7-8 blocks might sound easy but when it is 95° and 100% humidity, it will make folks cranky really quick.
I would also suggest a Duck Boat tour - being a local, my out of town family wanted to do it and I was dreading it but it was actually very good and I learned a thing or two when doing it.
Just remember when going to Wildwood how far the beach is from the boardwalk to the water...Ocean City might be better for the beach aspect and Wildwood later on for the rides and the boardwalk (and I actually don't mind OC's boardwalk but then again, I don't play the games or ride the rides!)
But also take into consideration what the girl's interest is...if they like history then the Old City stuff will appeal to them (and everyone should see the Liberty Bell once in person) but if they like science, then the Franklin Institute or the Museum of Natural Science would be a great thing to see.
If they like art, Philadelphia's art museum (along with many other museums) are home to some wonderful works of art. The zoo can also be a good side trip...especially since it is the country's oldest...
Also, if they like the gross stuff (you never know with kids these days), the Mutter Museum is a great showcase of medical odities and general physological items dating from the 1800's that started as one man's personal collection.