What do you do on the islands in Pokemon FireRed?

Answer 1

The Sevii Islands are part of an optional sidequest that gives the player access to capturing pokemon that normally dont appear in the Kanto map, also completing the Ruby quest on One Island will let you access islands 4-7, lastly finishing the Sapphire quest on Six Island will grant you the ability to trade between the Hoenn Pokemon Games (Ruby, Sapphire, Emerald) along with Pokemon Colosseum and XD Gale of Darkness.