It will be dead. September is the slowest time of the year and there may even be some places that will be closed. You may have to wait until you arrive to find out what is going on, as there will honestly be very little. It will rain daily and it is the height of the hurricane season in September so expect storms.
Now having said that, yep, I go in September and adjust. I go anytime that I can fit it into my schedule for as long as my schedule will allow.
As far as YS, Black River, and Pelican. Have done these three in one day a few times. No problem. However, in September it will rain while you are at one of these places (or all three) so just be prepared.
