For upscale dining i'd suggest Pod or Morimoto's for asain/sushi, la Famigia on front street for italian (the wine cellar is amazing), le Bec-Fin for frenchâ€â€there are to many good places to list, what is your favorite kind of food?
for casual, Tony Lukes for hot sandwiches like cheesesteaks and esp. roast pork with broccoli rabe and sharp prov (its to die for) and Ricci Bros hoagies on s. 11th for the best hoagie of your life, they make jersey mike's (which i like) seem like subway.
You should also hit the itialian market at 9th and Passyunk. when you are there make sure to stop in at di Brunno Bros and get some food or just to taste some good cheese and get an espresso.
After you have eaten your fill you will want to go to get some gelato at Capogiro at Samson and 13th (i think?) its some of the best stuff I've ever had in my life. then you can walk right up the street about 4 blocks to Chris' Jazz Cafe or Zanibar and hear some great music.
If you are a beer drinker you should check out the Standard-Tap in Northern Liberties (just north of the Vine st. Express way). They have something like 20 local Pennsylvania beers on tap.
I lived there for a long time and go back to meet with clients or shoot about every 6 weeks, so if you want more suggestions or are looking for something specific let me know.