Center City-ish Dinner & Brunch Recs?
Honestly there are so many casual italian/seafood/great burger places in Philly. Try browsing menupages' list of restaurants
and narrow the list down by cuisine, feature (casual, byob etc) and price range. When you see a restaurant that looks interesting either check it at yelp.com or use the search function here, it'll save a lot of back and forth posting about resto's that have already been written to death about. Apologies if I sound dismissive, but you'll most likely find what you're looking for much easier and quicker.
The only caveat is that for brunch many places have a serious (1 hour+) wait on the weekends, so either try something off the beaten path (like Fitzwater cafe on Fitzwater and 7th, or Distrito, which is pretty quiet for brunch) or call ahead and make sure the wait is bearable. Enjoy your trip!
I second the recommendation to pick a not-as-popular spot for brunch so that you can actually get in for brunch before dinner time. I recently ate at Delicatessen at 703 Chestnut for brunch, it was spectacular and was nearly empty. It's sort of a modern take on a Jewish deli. My "philly bagel roll" was amazing: toasted sesame seed bagel, wasabi cream cheese, smoked salmon, pickled onions, cucumbers, and tobiko. coffee was good and strong and service was friendly and quick. It's on your way from your hotel to the Independence Mall area as well.
For other foodie-quality casual spots in Center City try a gastropub like Good Dog, Pub and Kitchen, or Varga Bar. For burgers, I've heard good things about the new 500 degrees on Sansom Street.
For Italian, my favorites are farther into South Philly like Le Virtu (get a reservation), but a little closer is Dante and Luigi's, it's old school Italian, cash only, and a reasonable walk from your hotel.