Jump rope mini-game
Have Vivi approach the girls jumping rope near the ticket both in Alexandria. Talk to them and they will let Vivi join them. Press X when the symbol appears over Vivi's head to jump. Keep jumping successfully to get Gil, rare cards, and other items.

Two-player battles
In the config menu under battle control, select custom and you can have a second player command take command of selected characters during combat.

Other endings
End the game with the "Zeus Hammer" and/or the "Save The Queen" in your inventory to get additional endings.

Infinite MP
Have Amarant in your party and make sure he has learned the Chakra skill. Also, the minimum MP needed for this trick to work is 4 MP. Enter a battle and have Amarant cast Chakra on himself. For 4 MP, he will restore MP equivalent to about 20% of his maximum MP. Continually cast it until you have reached your maximum MP. You can also do this to all other characters in the party as well.

Easy AP
Walk around in the darkness for awhile in the field and forests around the town that has the auction house and lady who collects the zodiac signs. You will encounter a ghost. Give it some ore and you will win the battle, resulting in 10 AP.

Easy experience
Use Quina's level 5 Death spell against the level 60 Grand Dragons on the outside of Gizamalike's Grotto for around 8,800 experience per person. Equip the quick level gain support ability (level up) for even more. Also, remember to take Coral Rings to protect against the Dragon's Thundaga spell.

Black Jack mini-game
 

Successfully complete the game. Once the credits finish and "The End" screen appears, press R2, L1, R2, R2, Up, X, Right, Circle, Down, Triangle, L2, R1, R2, L1, Square(2). If you entered the code correctly, you will hear a sound. Press Start to begin playing the Black Jack mini-game.