Flygirl - On the high end - I agree about the Striped Bass and I would add the following.

I would also recommend the Saloon in South Philadelpia
750 S 7TH St
Philadelphia, PA 19147-2944
Phone: (215) 627-1811

and of course (very high end)

Le Bec-Fin (Center City)
1523 Walnut Street
Philadelphia, PA 19102
Phone: (215) 567-1000


Just hanging out during the day

Reading Terminal Market - Various choices and all are good.
11 and Arch streets
Center City Philadelphia - next to the convention center.