This page contains links to the main secrets and sidequests of Final Fantasy X. Other than the Cavern of the Stolen Fayth (between the Calm Lands and Mt. Gagazet), none of these quest will be fully open until you have acquired the airship (after your visit to Zanarkand Ruins). You will be partly able to complete some of the Monster Arena and Remiem Temple when you reach the Calm Lands (which is recommended), but you can only fully complete these quests with the help of the airship.
From StrategyWiki, the video game walkthrough and strategy guide wiki