Are you 100% on the fulvous duck sighting? They are indeed around S. Florida and have been at Green Cay before, but the black-bellied whistlers are much much more common, and fulvous sightings there tend to be rare. There are even a few fulvous/black-bellied whistler hybrids around, including one that has been at Green Cay and Wakodahatchee the past few years!
If you've been to Green Cay and Wakodahatchee, you should have seen a few martins and swifts, which are omnipresent there. In Boca Raton, you can often catch a variety of terns and some waders behind the library on Spanish River Blvd...the lake there is popular with terns, gulls, and osprey.
Also, hawks have been at Green Cay the past few weeks - I've been regularly spotting a red-shouldered hawk, including this past Saturday, and I have a cooper's hawk still daily invading my backyard. Spanish River beach is decent for shorebirds - a few gulls would likely be there too - also, Boynton Inlet tends to have a fair number of birds because of the boat basin there and the fish cleaning tables...gulls, pelicans, etc are usually present.
At Green Cay and Wakodahatchee, the least bitterns have been very plentiful this year - you should have spotted those. And black-crowned night herons are usually there too, if you haven't spotted those yet. I've been surprised at Green Cay for the last two weekends in a row to have photographed killdeer - they used to be common there, but it's been a few years since I last regularly saw them, and now to have them present in summer is surprising!
BTW - I'm at Green Cay and Wakodahatchee every Saturday, sometimes on Sunday...so if you're in the area in the early afternoon, let me know - I'll be there.