Without any more details, I'll give my favorite restaurants to recommend. I don't go to Rittenhouse as often as other neighborhoods so I've only been to a dozen or so restaurants there. I live in Fairmount, but I don't typically send travelers up my way, though there's a ton of stuff to do and great places to eat - Sabrina's for brunch, Pizzeria Vetri for the best pizza in town, La Calaca Feliz is the best Mexican, Figs is great, Hickory Lane is great.
Rittenhouse Square
Vernick (agree totally with DaDeacs)
Twenty Manning
Sbraga
Pumpkin
Pub and Kitchen
East Passyunk (cab ride probably, but the hottest food area in town)
Noord
Fond
Will
Stateside
Society Hill/Old City/Washington Square
Ela
Zahav
Talulah's Garden
Amada
Rosa Blanca
Barbuzzo
Zinc
Tinto
If you do go to Reading Terminal for lunch, and you've never had a Philly style roast pork, go to Dinic's, get a roast pork, sharp prov, broccoli rabe. Otherwise if you feel like traveling the next best roast pork is at John's way down in South Philly. Or for a great lunch sandwich selection, go to Jake's Sandwich Board for huge, unhealthy sandwiches. Sonny's for cheesesteak (skip Pat's/Geno's) followed by Jim's.
I'd recommend places like Laurel, Vetri, or Morimoto, but they're prohibitively expensive and hard to get into.
I have a tough time narrowing down this list, but neighborhood agnostic, my top 3 recs are Ela, Vernick, and Sbraga.