I have a couple of friends who are planning a trip to the City of Brotherly Love and they are asking me for advice. But I haven’t lived there for 7 years and haven’t been there in 2 so I might not be the best person to ask. All I’ve been able to come up with is the Mutter Museum, South Street, Independence Hall and the Liberty Bell. Where else should I send them? I know I am forgetting something. Help me please.
Franklin’s house (Market between Third and Fourth)
The Market East area – Market between Front and Fifth or so – has turned into kind of a trendy little restaurant area – couple of nice used bookstores on N. 2nd
Rittenhouse Square area; Art Museum; Franklin Institute; University Museum; Academy of Fine Arts…
If they’re interested in history and government, the National Constitution Center is fun.
Robin
The Franklin Institute, the Art Museum (you know they want to do the Rocky thing), the Academy of Natural Sciences, Reading Terminal Market, the Italian Market, the Mummer Museum, Independence Hall, and one of the walking tours of Old Philly. I heartily recommend the Constitution Center. I think the Liberty Bell is overrated.
The mall between One and Two Liberty Place (15th and Chestnut, I think?) is just a mall, but with skyscrapers.
Send them to Eastern State Prison. That’s supposedly an entertaining tour. Not a big fan of the Constitution Center, but since its right in the middle of all of the Independence stuff, you might as well go there.
They could cross the bridge to see the acquarium in Camden if they are into that.
Check out the Philadelphia Zoo. No, not the one at Broad and Market with John Street, et. al., the one at 34th and Girard with all of the nice critters. 
Also, Firemen’s Hall at 147 North 2nd Street is the place to see the history of firefighting dating from Benjamin Franklin’s founding of Union Fire Company in 1736.
Eastern State Penitentiary is closed for the season – they won’t reopen till April.
And :smack: on forgetting Reading Terminal Market – that’s where I always take people for lunch when I’m giving them the one-day tour of Philly.
Yes, the Franklin institute is a must (I can’t count how many times we went there on school field trips, and I still yearn to go). Make sure you walk through the big heart and ride the train! Walking down South street in the vicinity of Jim’s Steaks is also cool. 
Haven’t lived in Philly for many, many years, but there is so much to see & do not just in the city, but in the region. The Rodin Museum, which is also on the Parkway (along with the Art Museum and the Ben Franklin Museum), might be an interesting side-trip. Someone mentioned the Aquariam in Camden, which is just across the river from Penn’s Landing, I think. Region-wise, there’s New Hope in Buck’s County - an artsy-craftsy town (a bit expensive too, I think); to the south is Longwood Gardens (a former DuPont estate turned into a botanical garden), all decorated for the Holidays. Hope this helps give some more ideas! 
Get a cheese steak at Pat’s…but thos egoes without saying, doesn’t it?