There's a surprising amount of interesting things to do in Final Fantasy X but it's not always obvious how to find them. In this top 10 list, I'll break down my favourite of these nifty secrets; perhaps you'll discover something new!
Al Bhed Primers
Al Bhed Primers are hidden books that teach you letter translations of the Al Bhed language. The first one is located on the ship where you initially meet Rikku behind some Al Bhed. The books are kind of hard to see so you really have to keep your eyes peeled. After you collect them all, Rin will give you 99 Underdog's Secrets which is a Customise ingredient for Double Overdrive.
Tidus is the only character who unlocks new overdrives just by using his current ones while Wakka's are prizes from Blitzball, Auron's are rewarded for finding Jecht spheres around the world, and Kimahri gets his overdrives from his Lancet ability. Unfortunately, Rikku, Lulu, and Yuna don't have any additional overdrives.
The Celestial weapons are the strongest weapons in the game and there is one for each party member. First, you'll need the Celestial Mirror to create celestial weapons. To get it, you'll need the cloudy mirror found at Remiem Temple after speaking to Belgemine. Then, you'll have to bring it to Macalania Woods. Next, you'll have to do a short side-quest where you have to find a kid's father then find the kid who wanders off. Finally, there's an orb structure where you create the weapons.
Each weapon has a crest and a sigil and you also need to find the weapon itself. For example, Wakka's weapon can be claimed in the bar in Luca after you return with the airship. Then, pick up the Jupiter Crest in the Aurochs' locker room and get the Jupiter Sigil by playing Blitzball. Finally, return to Macalania Woods with the Celestial Mirror as well.
Remiem Temple is accessible in the southeast part of the Calm Lands but you need to tame a chocobo to get there. Just outside the temple, there's a chocobo racing mini-game where you can race a chocobo while picking up treasure chests for various prizes. Among the prizes are 30 Wings to Discovery, 30 Pendulums, and 60 Three Stars.
Inside the temple, you can fight the summoner Belgemine for some very useful ingredients used to teach your Aeons new spells. Behind Belgemine, there's also a Fayth to obtain a new Aeon but first, you need the Flower Scepter from defeating Belgemine's Bahamut and the Blossom Crown from capturing one of each fiend on Mount Gagazet then speaking to the Monster Arena owner.
Cavern of the Stolen Fayth and Yojimbo
You've likely visited the cave in the valley before you went to Mount Gagazet but if you haven't, there's a secret Aeon there: Yojimbo. However, make sure you're prepared to pay at least 250,000 gil for his services before attempting to recruit him.
Baaj Temple and Anima
Once you unlock the airship Fahrenheit, find the co-ordinates X: 11-16, Y: 57-63. That's in the southwestern ocean west of Besaid. While there, jump down into the pond where Tidus fell in the beginning of the game and you can refight the swimming skeleton Geosgaeno. Only Tidus, Rikku, and Wakka can fight this boss because it's underwater. Be warned that it has a stone punch attack that will shatter your party members if you don't have stone ward (which is 100% effective in this fight) even if they're not petrified.
Past Geosgaeno, there's a Fayth for the Aeon Anima. First, you'll need to collect treasures from 6 Cloister of Trials: Besaid, Kilika, Djose, Macalania, Bevelle, and Zanarkand. Beware of the Dark Aeons if you plan on returning to Besaid or Macalania temple. Also, make sure to collect Lulu's celestial weapon (the Onion Knight) from the pond.
Using the airship, head to co-ordinates X: 67-75, Y: 33-38 which is somewhere in the Eastern ocean. Omega Ruins is a dungeon filled with some of the toughest monsters in the game and they give the best AP as well. At the end is the boss Omega Weapon which is also one of the toughest bosses in the game. If you want the most items, make sure you steal from it 3 times.
Monster Arena and Nemesis
The Monster Arena owner has you travelling the world capturing fiends from every area you visited but you have to use special capture weapons sold by him. One of every fiend should unlock most monsters in the arena but if you want to unlock the ultimate superboss Nemesis, you need to capture 10 of every fiend and then defeat all of the Area Conquest, Species Conquest, and Original monsters. The monster arena bosses are nearly the hardest bosses in the game and can even surpass Omega Weapon. They're also the only way to obtain sphere grid altering spheres like HP or strength spheres. You'll need to obtain plenty of these spheres to take on...
You may have encountered some of these accidentally if you wandered around like me. If your defense stats are low, they'll easily deal 99,999 damage with their attacks. They also have overdrives like any Aeons that can wipe out your whole party. Only challenge them if your stats are extremely high. Auto-phoenix armor is extremely useful, too. Here's a quick list of their locations:
- Dark Valefor: Besaid Village
- Dark Ifrit: Bikanel - speak to someone near where you entered Home in the middle of the game
- Dark Ixion: Thunder Plains - speak to a warrior monk standing under a lightning tower just north of the agency
- Dark Shiva: Macalania Temple
- Dark Bahamut: Yunalesca's chamber in Zanarkand
- Dark Yojimbo: Cavern of the Stolen Fayth - take the teleport pad to the end of the dungeon and walk backwards
- Dark Anima: redo the trials in Mount Gagazet cave and he should appear where you fought Seymour
- Dark Magus Sisters: enter Mushroom Rock Road from Old Road
Penance (the true final boss)
After beating all of the Dark Aeons, Penance will appear above the Calm Lands and then you can travel to Penance using the airship. If you've unlocked Penance, you must already be extremely strong so all I can say is... good luck!