A less popular NPS location by Independence Hall is the 18th Century Gardens. Its smaller than the other locations that I mentioned but its much more intimate and actually works nicely for smaller ceremonies which I've seen in the past. Its a nicely manicured garden with a gazebo'ed backdrop. It works well as a 2nd location if you wish to do more than one NPS location under your permit or if you are looking for a location to do an intimate outdoor ceremony in downtown Philly.
*this location requires permits through the National Parks Service and runs typically around $175. NPS books in 1 hour periods and they give the option to do multiple locations under the same $175 permit.