I live in south jersey about 20 minutes from downtown philly and i go their about 2 times a month, so i know my way around. Below i listed the top 5 "must sees" in philadelphia.
Go to Jim's Steaks on South Street in south philly. Philadelphia is famous for their cheese steaks and Jim's is definitely a place for that. Geno's Steaks is also another famous Steak place downtown as well as Pat's, but my favorite is Jim's for sure. Never leave Philly without a cheese steak!
Go to the Frankin Institute downtown. It is an amazing museum especially if you have kids, but if you don't its still really cool!
A very romantic place would probably be at the LOVE circle i guess you can call it. It is like a park and you can eat lunch and their is this sculpture that says LOVE.
A very fun place to visit is Dave and Busters. It is an arcade place but they also have bowling, billiards and a resturant. It is right along Pennslanding, you can't miss it!
And last, but not least you have to take a ride on the duck. You get on along Pennslanding and it is kinda like a boat on wheels and it takes you through a tour of the city and it is called " Ride the Duck" because it gives you a duck beak whisle thing that people blow, its really funny you can hear it from really far away. After it is finish taking you through a tour of the city it takes you in the water on a boat and sails around for a little while, it is great especially if this is your first visit to philadelphia and it will explain a lot.
Have a great time in the city of brotherly love, and make sure you visit those places!