Best place for waterfowl would be Heinz NWR (a.k.a. Tinicum) which is right next to Phila. Airport.
The largest variety of gulls would probably be in the area of Tullytown Landfill (on the Delaware River northeast of Bristol), but other than ring-bills and American herring gulls, most will be species that occur in the UK.
Middle Creek WMA north of Lancaster would also be good for waterfowl, especially for snow geese and tundra swans. I don't know if the lake is unfrozen yet, though.
The sheer number of bald eagles at Conowingo Dam on the Susquehanna River in Maryland just across the border from PA is impressive, although the variety isn't the same as the other locations.