Have a look at this site for some suggestions on locations, he also has a second visit report on his site.
I reccomend the Tavernier Wild bird rescue centre for close ups of the Pelicans.
We did the keys & the West coast up to Cape Coral during May & had a great time. I recommend the Everglades & also the "Loop Road" Both of which are an fairly easy drive.
I took my 500mm as I flew with BA. They do not have a weight limit on hand luggage.
Thanks for the information, I'll check out your links.
I'm interested that you took your 500, did you also take a tripod or monopod?
There is a little tidal swamp just North on US-1 outside of Marathon on Grassy Key, it has been very hit and miss for me, but at the right times I have seen dozens of egrets and herons, ibis, and kingfishers there. Other times it has been totally empty. It should be a very short drive for you, so worth checking on:
(let me know if the link doesn't work right).
Too bad you aren't coming the last week of September, that is the dates of the Florida Keys Wild Bird Festival: http://keysbirdingfest.org/
As mentioned above, if you want a close up view of some captive (or wild but highly human friendly/fed daily) native birds of the Keys, the wild bird center in Tavernier/S. Key Largo is a good stop. They do not charge for entry, but are a charitably operated organization, so please leave a donation.
Also, if you want a close up of some pelicans or egrets, the fish cleaning tables at most marinas with charter boats tend to have a very well tempered bunch of birds who come every day for a free meal.
Thanks Bill, the link works fine. It's a shame to miss the festival but we are stuck with our dates.