Hey fellow Final Fantasy X explorer! When it comes to side quests, there are a few key ones you definitely don’t want to miss.

1. **Monster Arena**: By capturing fiends throughout Spira, you can unlock powerful new monsters to battle in the Monster Arena. Not only is this a great way to level up and earn rare items, but it’s also necessary for unlocking some of the best equipment in the game.

2. **Celestial Weapons**: Each character has a unique Celestial Weapon that is incredibly powerful when fully upgraded. Be prepared for some challenging mini-games and boss battles to unlock these ultimate weapons!

3. **Dark Aeons and Penance**: After completing the main story, you can challenge the Dark Aeons and the super boss, Penance. These battles are some of the toughest in the game but offer great rewards for those who are up to the challenge.

Remember to explore every nook and cranny of Spira to uncover all the secrets and hidden