Center City has a ton of restaurants that offer outdoor seating. Some are wonderfully placed with table settings and whatnot. Others clutter the sidewalks and make it unattractive to sit and enjoy your meal. There are a couple of spots that you should check out.
Tria in Washington Square West is a lovely restaurant that serves some delicious wine and beer. If you are a breakfast lover, Beau Monde is a wonderful french restaurant in Bella Vista with, what some will say, the city's best crepes. Finally, Cafe Ole in Old City is a beautiful coffee shop style restaurant with delightful seating. All three offer something different.
Another suggestion is to take a stroll through Center City. You are guaranteed to come across at least one restaurant with outdoor seating, especially during the Spring and Summer. Happy Eating!