The "Resort Town" island is available to you once you win your gym battle against Skyla. There are lots of tropical fish to catch here in the water and tall grass. Also in the buildings are special people who can do things like help your Pokemon increase in happiness, Pokemon massages and a restaurant that serves Pokemon stat boosting food.
There probably will be some kind of story element here as well. I mean without one there is really not much initiative to figure out you can fly here. Enjoy your stay! Map & Effects (c) Me