Philly has some excellent museums: favorites of mine include the Phila. Museum of Art, Barnes Foundation (stupendous, and will take several hours), Rodin Museum, etc.

Some good places to walk around on a nice day: Kelly Drive (start from the Art Museum and walk along the Schuylkill River to the north), Penn's Landing (a few blocks east of the National Constitution Center, along the Delaware River), South Street between 2nd-18th Sts. (a funky neighborhood with fun shops and restaurants), Italian Market, Rittenhouse Square.

Philly has very good restaurants all over the place, so if you are into food, do some research and you can have great meals while you're there.