A full list of the side quests that can be completed in the game:
Cavern of the Stolen Fayth
This side quest can be completed as soon as you reach the Calm Lands and is where you obtain the optional Aeon Yojimbo.
Airship Passwords & Hidden Coordinates
A list of the Al Bhed passwords and search coordinates for exploring Spira from the airship destination list.
Al Bhed Primer Locations
Make sure that you picked up all of the Al Bhed Primers scattered throughout Spira. Note that you may not be able to return to certain areas of the game to retrieve some if you missed them.
Add the [No Encounters] ability to your armor to prevent random encounters from occurring while traveling.
Information on how to obtain the Celestial Mirror and the Celestial Weapon for each character.
Obtain the Celestial Mirror and battle all of Belgemine’s Aeons to receive the Moon Sigil required for Yuna’s Celestial Weapon, Nirvana.
Village of the Cactuars
Complete the Cactuar Village side quest to obtain the Mercury Sigil, one of the pieces of Rikku’s Celestial Weapon, Godhand.
Obtain all three of the optional and secret Aeons in the game (Yojimbo, Anima and Magus Sisters).
Complete Baaj Temple to obtain Anima, one of the unlockable Aeons.
Obtain all of the Overdrives for each of your characters.
Battle against some of the strongest fiends and bosses in the game.