Whenever I go to NY by myself I spend a few days in Philadelphia as well. My friend Christina and her husband Adam live here with their little daughter Adelina. Christina used to live in NY and I have stayed with her many times in my favorite city. To get to Philadelphia from NY I take the Bolt Bus. A really easy and cheap way to go from NY to Philadelphia. The bus takes about 2 hours or less/more depending on the traffic.
Philadelphia is such a historical and beautiful city. It is a more "personal" city compared to NY as it is not so big, and the neighborhood where Christina and Adam live is filled with tall red brick buildings, where everyone has a decoration on the door and the American flag is waving in the wind.