I can't help you on a rental, but there are definitely places worth riding. Two particular areas I've been wanting to get my bike to are the Schuylkill River Trail and Valley Forge National Historical Park, which you can get to by bike via the Schuylkill trail.

The times I've been to Philly, there've been bikes everywhere so the area definitely seems to have a healthy cycling culture. Not to mention the fact that they host an international pro race every year. Unfortunately, I go up there to visit a non-cycling friend and haven't been able yet to arrange the time for a ride. I had planned to head up there with my bike today on my first day of vacation, but it's pouring down rain here in the mid-Atlantic