This area is home to many of the most elegant restaurants in Philadelphia [1], but you can also find anything in this area.
Walnut Street is lined with restaurants between Broad and 18th Streets, with additional options on the surrounding side streets, including several directly on Rittenhouse Square [2] and a few good options on 20th Street.
Links:
[1] http://www.moon.com/destinations/philadelphia/discover-philadelphia
[2] http://www.moon.com/destinations/philadelphia/arts-and-leisure/recreation/parks-and-gardens/rittenhouse-square