Silver Chocobo:You must breed 2 S class Gold Chocobos withthis hidden nut.
Revive Aeris:You must somehow get her white materia.
Ultra Materia:You must master every Materia and ride a Silver Chocobo underwater and throw it on the ocean floor and go back to the Surface and a ball will come flying out of the water.
Outer Space:You must Find the Secret Entrance to the Rocket In the Crater and find out the Password.