What are your interests? Historical spots it has in spades, museums, parks, theatres. If you can handle crowds the Reading Terminal Market has all kinds of food; but it's very popular and next to the convention center so every time I've been it's crowded, like wall to wall. Must have's food wise is the soft pretzel and a cheesesteak if you've never been before

Otherwise it's been a while since we've eaten in the city so I'm not even sure if some of the places are still around or if they're as good as before. We only go down for dim sum now cuz that's the one place we haven't been able to find in the 'burbs
If it's the first full weekend of the month, and you have either credit or debit from BoA, they have free museum entrance. The art museum (the steps from the Rocky movies) and the Constitution Center are on the list. (
https://about.bankofamerica.com/en-us/what-guides-us/arts-and-culture/partners.html#fbid=cQMKcBNXIs2) If you have littles still the Please Touch Museum gets really good reviews (and does look fun), for all ages the Franklin Institute (as adults we still go!).
Even if you're not into historical stuff I think everyone should see the Liberty Bell at least once in their life. (yeah it's just a bell, with a crack no less

, but it's The bell, ya know?)
Fairmount park is great for nature stuff, trails for walking or biking, plus it's near Boathouse row which is kind of iconic. I think they have a trolley tour or something for the mansions within the park and they have an outdoor concert space.
If you have a general area you planned to stay or particular interests and I can narrow it down. Good place to start looking is the HOHO bus routes, that pretty much hits a lot of the highlights. So even if you don't have the time for the HOHO it at least gives a list of popular attractions you can dive into. We could let you know what's worth it or if there's alternates, but helps to know what interests you about a city.