Race Street Pier is fairly new and has a lot of spots that work well right within this tiny urban park.  Of course you have the Ben Franklin Bridge to work off of but there are some nice seating options and greenery to work with as well.  Works well for every scenario.  Engagement session, wedding parties, and even ceremonies too.  Works well with locations on the east side of downtown.  I've had nice luck with using Elfreth's Alley as a 2nd location along with Race Street for engagement sessions.

*Permits are required for this location through Delaware River Port Authority but its very simple to get and its only a $50 permit.