Philadelphia Museum of Art
The Philadelphia Museum of Art is one of the best you can visit in the city, if not the best. There are authentic gems in every corner and hallways of the museum, so it warrants a guided tour so that you don't miss a single detail.
Side note, the steps to the outer door of the museum were in the movie Rocky, so it's pretty normal to see at whatever time of day imitating the boxer climbing the stairs as fast as they can and air-punching once they reach the top.
At the foot of the steps there's a statue of Rocky Balboa that draws something of a crowd.
It's because in its day, as thanks to Stallone for having shot the movie there they placed the statue in the entrance of the museum, but later on it was decided that such a statue didn't warrant being put near works by Van Gogh, Picasso and the like, so they moved it away.