The Water Works area behind the Art Museum is another popular spot and a frequent spot for sessions that I do around the Art Museum area.  There is a bunch of space out here but the popular space in this vicinity is out on the lookout point by the waterfall that looks over Boathouse Row.  Spin around and yet another great view of the Art Museum and the Philly skyline.  This area can be busy on weekends but during the week its not as much of an issue.