Pietro's has great pizza & salads (121 Sourh Street). Also, if you like cookies/eclairs, stop into Famous 4th Street Deli (4th & Bainbridge) for a great chocolate chip cookie! (they do make huge deli sandwiches too if you like that for a casual dinner- and a free mini cookie, but the real ones are better IMO).
Definitely hit the Old City suggestions listed. If you like Cuban food Cuba Libre is also nice for a great mojito!
You are also in easy walking distance of South Street (there is a pedestrian overpass). Near there are Ansil (3rd and Bainbridge):innovative small plates, Gayle (3rd between South and Bainbridge) deconstructed dishes, Xochitl (2nd between South and Pine) upscale mexican, serviche, Southwark (4th and Bainbridge) old fashioned cocktails, intelligent bar discussion and good american food, and newbie Coquette restaurant and raw bar (5th and Bainbridge) french Bistro.
I could go on...post again if you need more info!
You'll be in walking distance to Old City, which has many wonderful eating options. My suggestions are:
Zento for sushi, 138 Chestnut St (BYO, there's takeout beer available at Rotten Ralph's, 2nd & Chestnut, and a liquor store w/ wine, sake, etc. a half block north on 2nd St.)
Mandoline, 213 Chestnut Street (BYO)
Amada for tapas, 217-219 Chestnut Street
Bistro 7, 7 N 3rd Street (BYO). I've enjoyed dining alone at the chef's counter many times.
Society Hill Hotel Bar, 301 Chestnut Street. Great upscale pub fare, friendly bartenders and patrons.