We spent a day there in August. It was at the tail end of our NYC trip and we took an Amtrak train from Penn Station to 30th Street Station in Philadelphia (not something I will ever attempt to do again with 4 young kids and luggage for 6, a car seat and stroller - using public transportation- but very doable for two adults on a day-trip). I think what did me in was my husband insisting we walk to our hotel from the subway system with all of that in tow. And it was a lot further than he claimed it to be. Anyways, I am getting off track here.
Once we got settled in our hotel, I really LOVED Philadelphia. We walked over to South Street and got ourselves a cheese steak sandwich and italian ice for dinner. The next morning we walked over to all of the historical sites and saw the Liberty Bell, Independence Hall, etc. One thing I did want to mention is, there are certain buildings like Independence Hall that you need to get tickets in advance (free) in order to get in. If I remember right, they recommend you go to the National Convention Center first to get your tickets. We didn't really plan ahead so there were things we weren't able to see because we didn't get tickets, plus we were really limited on time. They also offer the Duck Tours. We saw lots of people doing that too. I really wished that we had at least one more day to explore and see things. Everyone was so friendly too.
Hope you have a great time!