The Waterfront that overlooks The Ben Franklin Bridge, Schuylkill River and Camden, NJ has to be my favorite part of Philly. The area is close enough to the downtown for dinners and lunches, but far enough to enjoy the cool river breeze and a brief relief from the city.
My favorite part about the waterfront, especially during the Summer months, are the events... most of which are free. They have movie nights on Thursdays (I have to make it to at least one) and ethnic events on weekends (I look forward to the Hispanic Festival every year).
Furthermore, it's great for people-watching, as there are so many beautiful people and families browsing around at all times.
If you guys ever visit Philly, grab some ice cream from Franklin Fountain or an icy drink to keep you cool and hydrated and walk around.