Special Airship Passwords
When the airship becomes accessible, you can use the "input" command and enter some passwords that allow access to certain areas where you can find special weapons. Enter "GODHAND," in all caps, to get to an area containing Rikku's legendary weapon. Enter "VICTORIOUS" to get to a location with armor for Rikku. Enter "MURASAME" to the sword of the same name for Auron which comes with the One MP Cost ability.
Secret Airship Coordinates
When you speak to Cid aboard the airship and he shows you the Map menu, select "search," where you can enter coordinates to find secret locations. To find Baaj Temple, enter "X:11-16" and "Y:57-63"; to find Sanubia Sands, enter "X:12-16" and "Y:41-45"; to find Besaid Falls, enter "X:29-32" and "Y:73-76"; to find Mi'hen Ruins, enter "X:33-36" and "Y:55-60"; to find Battle Site, enter "X:39-47" and "Y:56-60"; to find Omega Ruins, enter "X:69-75" and "Y:33-38."
Earn Gil Quickly
The enemy "Mimic," found in the Omega Ruins, gives 50,000 Gil after you defeat it. If you defeat it with the "Gillionare" ability, you will get twice that amount.
Lots of AP
During the first battle with Sin on the boat, attacking the sinspawn rather than sin's fin provides a great deal of AP with the more of them that you defeat.
Potions
Early in the game, when on the Al Bhed boat for the first time, go to Rikku, who will give you three potions. Dive into the water and swim into the next area, swim back and get three more from her. You can repeat this until your inventory fills, to keep or sell.
Don Tonberry AP Trick
Fight a Don Tonberry at the Monster Arena with some special conditions. Equip an attacking character with a weapon containing Triple AP, Triple Overdrive, and Overdrive -> AP, and turn his overdrive mode to "stoic." Turn the other characters' overdrive modes into "comrade." Attack the Don Tonberry repeatedly, who retaliates with an attack called "karma," which will likely kill the character, so raise the character repeatedly. After the Don Tonberry gets close to the party and stops casting karma, flee from battle. Due to the weapon configuration, the attacking character may likely reap over one million AP.