What to do for the day in Philly?

You could stick to The Liberty Bell and Independence Hall, both are free and would be a good part day activity. Philly is very walking friendly and you'd be able to walk from your hotel to Old City very easily. In both those areas there are a lot of good food options too. Old City has a lot of local restaurants and chains like Cosi. For a park I would recommend Spruce Street Harbor Park which is only a few blocks from The Liberty Bell. A lot of Philly chefs/restaurants have stands there that offer food and beverages. Check out Philadelphia's Magic Gardens on 10th and South if you have time. It's a unique place that kids would find interesting too. 

This time of year the place to be for outdoor scenery is the waterfront along Columbus blvd. There's the Spruce Street Harbor Park, Blue Cross RiverRink and Morgan's Pier just to name a few. Right now The Franklin Institute has the Genghis Khan exhibit. The Philadelphia Museum of Art is always good for tourists if not only for the rocky steps. Also look into one of the adult walking tours because philly has an interesting and tattered past. If you're wondering about food, just eat something because really everything is good and your hotel is right by restaurant row which is full of great choices.

I can't believe no one has mentioned one of the most popular things to do in Philly! Take a stroll through Love Park, we're famous for it :) Spruce Street Harbor Park is a really popular area, it's great during the summer since it holds a ton of outdoor events. Longwood Gardens is so pretty to walk through, slightly outside of the city, but worth the drive. Mütter Museum is by far the most interesting museum i've ever been too... weird medical oddities. 

Ride The Ducks Philadelphia (6th and Chestnut) is a great way to see the city and point out the highlights.