In the last year I visited the Ilha Comprida 4 times and I experienced a lot of it and of the places around it. I just have to say that the place is like paradise. As far as I am concerned, you might call the unique nature, with rainforest on the mainland and one of the biggest beaches on the island. You could also refer to the unique dune area on the island.
For you are describing the whole area, I really miss a recommandation to visit the town of Cananeia! And if people want to go there and talk to locals in English, let them visit the bistro “Farolito” on the Avenida Princesa Isabel 767 in Iguape. The owners speak very good English and they serve splendid food. Last time I was there (last February) they planned to have some guestrooms too!
Also call Iguape the capital of “Polo ecoturistico do Lagamar”. Their office on the Rua XV Novembro 131 in Iguape is also a place of exhibition and they welcome everybody to visit it.
And also good to know: Lots of fishermen will be happy to take people for a tour on the Ribeira for two hours, four hours or an entire day. You will see overwhelming nature and many many animals, especially birds and, of course water wildlife!