There is 4 things you can do on this island: Capture new pokemon, Train your current team, Try out Trainer Tower in the northern region of the island, Solve the Tanoby Key puzzle to obtain the pokemon Unown from the Tanoby Chambers. Trainer Tower is optional if you can complete all the floors you will be rewarded with nice items like king's rock, Tanoby Key is a cave hidden nearby a Cooltrainer couple if you can solve the boulder puzzle inside you can capture the pokemon Unown inside Tanoby Chamber which are set at the southernmost part of the island.
In icefall cave on 4 isle.
5 isle w/super rod
Seven Island
Island Seven. In the ruins.
It is on Seven Island, to the north.
Seven island at sevault canyon.
first get them on firered/leafgreen from the seven islands, then migrate them
northern seven island
beat all the gym leaders
Cinnibar island if your going in order.
Cubones can be caught on Seven Island.
The strongest wild pokemon can be found in either Cerulean Cave or Seven Island.