Oddly enough, I'll be out in your neck of the woods in May (for the first time). Staying in Napa and then in SF.
Anyhow... the Phila Zoo is (I believe) the oldest in the US. Something for the whole family. And, the Phila Art Museum is really a very wonderful place to visit - try and get there (and don't forget to do the traditional "Rocky" run up the steps!). Also nearby is a little know haunt, the Mutter Museum (for freaky medical oddities). You'll also want to take a walk through Reading Terminal Market, which is right by City Hall (the first "White House") It would be very beneficial for you to take the "Duck Tour" around the city.
As for restaurants, believe it or not, Philly has some of the best in the country. That's why we're so fat!
You HAVE to have a Cheesesteak. You MAY have thought you've had a Philly Cheesesteak before, but you haven't. There will be many people who tell you to go to Pat's or Geno's (they're right across the street from each other and overrated), but MY favorite is Jim's on South Street (around 4th St.). Do NOT ask for swiss cheese. Actually a "true" Philly cheesesteak uses (yep) Cheese Whiz. Mushrooms, while some will say it's OK, the true originalist will disagree. The only thing "allowed" on a cheesesteak, other than Whiz is fried onions, which I recommend. When you're in Jim's, check out the walls. They're adorned with thousands of pictures of famous people who've visited. Jim's is the REAL Philly cheesesteak. The proper way to order is "Whiz wit" (Cheesesteak with Cheesewhiz wit(h) fried onions).
And, while you're on the junk food kick... Tony Luke's for a hot roast pork with brocolli rabe. Another Philly favorite.
Fine dining... take a look for places up around Rittenhouse Square. There are quite a few places there. Plus, it's very historic. A great (and very safe) neighborhood - it's where the Philly "elite" call home. It's kind of neat to walk the streets and see the ACTUAL houses that historic people lived in!
In Center City, a couple landmark restaurants are Le Bec Fin (one of the best in the country), Susanna Foo's (also one of the best in the counrty)
Down in Old City (another wonderfully historic area), the oldest restaurant in the US, is City Tavern. Very nice - high ratings. Nor far from there is Betsy Ross' house, not to mention lots of other historic places, like the first mint, the liberty bell... another great area to tour! Another great restaurant around there is Buddakan.
I could go on forever. Hope you enjoy your trip! If you have any questions, or need some help or advice, PM me.