The Piazza in Northern Liberties is a cool little place. There are lots of boutique shops and a vendors market.
"PYT" serves big sloppy burgers-- among the best I've tasted in Philly.
"Darlings" is a classic American diner-- save room for their desserts
"Bar Ferdinand" definitely has the best food-- fresh and uniquely prepared. Ferdinand's is a tapas menu, which in this case means over-priced small portions served with lots of fu-fu waiter attitude. But every dish I tasted was "party-in-my-mouth" awesome.
"A Full Plate" is a very casual BYOB-- not much on ambiance, but the food is good and it serves large portions. We always leave with a doggy bag.
"Sonota" is a little bit more upscale, but it's still a moderately priced BYOB.
Restaurants to avoid: "El Camino" and "Appollonaire"
The Fairmount neighborhood has the Old Eastern State Prison, which is not your run-of-the-mill tourist attraction. I've taken the tour twice and the tour guide makes all the difference. (See if Bob is still working there. He's a great guide.) The Prison also has self-guided audio tours. As for nearby restaurants in order of preference: Illuminaire, Rembrandt's and New London Grill. And Jack's Firehouse is directly across the street from The Prison. Jack's weekend brunch is the best time to go.
Main St in the Philly's Manayunk section is a great to check out. It's like "South St" for yuppies (ie., Pottery Barn and Restoration stores). Main St has more energy on fall weekends, but it does host a few annual events in the summer which are well-attended. There are lots of shops and restaurants on Main Street.
My top choices: Bourbon Blue, Derek's or Manayunk Brewery (huge outdoor deck for dining over the canal water).
Enjoy your Philly visit!