The Reading Terminal Market. Hand's down the best place to graze.
The Mutter Museum rocks! But if you have the time, and the inclination, the University Museum at Penn has some of the best Egyptology exhibits on the East coast. At least they did when I was last there.
Rittenhouse square is also a must see, @ 19th and Walnut.
And you can't touch the Liberty Bell anymore
, but Independence Mall is worth a visit.