Kingdom Hearts Cheats

Easy mystery goo and items
Go to halloween town, and go to the bridge area. There might be rare truffles there. If there are, go down into the water, and go to the far right side of the screen. There should be one there. Cast aeroga or aeroa and knock it over to one of the hills. Just sit there while the sheild hits it up in the air, and recast aeroa or aeroga until you have hit it in the air 100 times.

Unlock the Ultima Weapon
After you have unlocked the Item Workshop in Traverse Town, keep synthesiseing all of the items. When you try to make the 25th item, it will be the Ultima Weapon (trust me, this isn't as easy as it sounds). The Ultima Weapon is the BEST weapon for Sora.

Unlock the Oblivion
In Hollow Bastion, go to where you fought Riku the second time. Look around to an upper level up there, then use Glide (an ability you unlock in the game) to get to the upper area. There will be a treasure chest containing the Oblivion. This is the second best weapon for Sora. This keyblade is a lot easier to obtain than the Ultima Weapon. If you can't get the Ultima Weapon, get the Oblivion.

Unlock many Mystery Goos
For this trick, you must have Sora's Trinity Limit ability. Go to any world where you have found some Black Fungus (mostly in Hollow Bastion and Halloween Town). Let Doanld and Goofy attack it, and it will soon turn silver and invincible. Get near Donald and/or Goofy and use Trinity Limit. It will kill the Black Fungus, releaseing some money and MP balls, then some treasure cheats containg Mystery Goo and Mystery Mold (you can sell mystery mold for 9999 munny). Use the Mysterty Goo to synthieses some items, or keep it and go get more. Very useful trick.