Elite 4
You must defeat the Elite 4 to travel to the rest of the islands. The Elite 4 are the final bosses, and they must be defeated in succession. The Elite 4 each have different types of Pokemon in the Level 60 range. Lorelei has Ice-type, Bruno has Fighting-type, Agatha has Ghost-type and Lance has Dragon-type Pokemon. Each of the four trainers will have five total Pokemon.
National Pokedox
You must capture at least 60 Pokemon to receive the National Pokedex, which you must have to get to the islands. This requirement is relatively easy, because Pokemon are all over the forests and routes. Once you have captured 60 Pokemon, talk to Professor Oak in Pallet Town. He will give you the National Pokedex.
Ruby
Talk to Celio in the Pokemon Center. Celio is the Non-Player Character who gave you the Tri-Pass. He will upgrade your Tri-Pass to the Rainbow Pass if you fetch a Ruby for him. Travel to the foot of Mt. Ember on the first island, and enter the cave. The Ruby is at the bottom of the cave. Return the Ruby to Celio and he will give you the Rainbow Pass. The pass covers four new islands, including the fourth, Floe Island.
Floe Island
Board the ship with your Rainbow Pass to travel to Floe Island. Pokemon can be caught in the island's waterways and its cave. There is also a Move Tutor on the island who will teach the "Body Slam" move to one of your Pokemon. Floe Island also has a daycare center to raise and breed different Pokemon.