Kingdom Hearts II/Halloween Town

First Visit
You begin the world in the Graveyard. Open a chest to the right for a Mythril Shard, then one to the left by the large gate for a Serenity Gem. Follow Zero into the Halloween Town Square for another scene that will place you in Dr. Finkelstein's Lab with new party member Jack Skellington. Open the chest for the Halloween Town Map, then exit to the Square for a short battle against some Heartless, Wight Knights in particular. Take note of the environment as well, as landing a stray hit on some landmarks will cause them to retaliate; the gate swings out at Sora, the guillotine drops, the grates launch acid upward (and Sora with them), and the fountain shoots acid. After the battle, open the chest by the guillotine for an Energy Shard and another by the attacking gate for a Mythril Stone. Return to the Graveyard and head straight to the Hinterlands. A chest behind a tree to the left contains an AP Boost. A chest to the right contains a Lightning Shard and another one up ahead contains a Mythril Stone. Examine the tree with the Christmas tree decoration to find yourself on Yuletide Hill in Christmas Town. Use the save point and moogle if you wish, then continue to Candy Cane Lane, where another Heartless battle awaits you. This one will be a bit more difficult, as it introduces a new, tougher enemy in the form of the Toy Soldiers. These look like harmless boxes until a toy soldier pops out to shoot Sora repeatedly, dealing significant damage with each shot. To counter this, either use Jack's limit or catch them in aerial combos.

Saving Christmas
After the battle, open the chest by the large double doors for a Lightning Stone. To the left of the entrance is a Mythril Gem. To the right of Santa's House is a chest by another house, and, lastly, the chest right next to Santa's House contains a Mythril Stone. Enter for a scene in which Sora receives some bad news, then open the chest to the right of the save point for an AP Boost. Open the large chest for a Christmas Town Map. Go deeper into the house to enter the Toy Factory, then follow the footsteps of Shot, Lock, and Barrel after the scene. Return to the Graveyard in Halloween Town, then take a left up the path and exit through the gate to enter Curly Hill for a boss fight.

Boss: Prison Keeper
Begin by performing aerial combos, and going into Valor Form if your health gets low or simply to speed up the process. The Prison Keeper has two powerful moves to look out for; the first has it twirl with its claws, creating a sharp whirlwind that can combo Sora into the air if caught. The second has it spin in mid-air while it disperses orbs of harmful energy in all directions. The former can be avoided by stepping back, though nailing the timing is difficult in this case, and the latter can be much more easily avoided by sneaking under it. It can also be caught in an air combo while it's shooting, effectively canceling the move.

Occasionally, the Heartless will look to the prisoners for aid and pour one at random into its mouth. This subtly changes its attack style, but the process of absorbing the prisoner itself leaves it impervious to attack. This is with the exception of the Inside Combo reaction command. However, though the reaction command does do some damage, it doesn't completely interrupt the process, and the Prison Keeper may still attempt to show off its new moves (such as throwing massive fireballs, which can be avoided by hiding behind the hill). Eventually, the Prison Keeper will swallow another prisoner in order to add even more attacks to its arsenal. Pay it no mind and stay on the offensive until its HP is drained to earn Sora Flash Step, Donald Hyper Healing, and an increase in maximum HP for Goofy and Jack.

After the battle, chase after the captive Santa and the newly-resurrected Oogie Boogie through the Hinterlands and back into Christmas Town. Go to Santa's House and into the Toy Factory to trigger a boss fight against a familiar foe.

Boss: Oogie Boogie
This battle has two steps to it. Oogie Boogie is on a high lift, blocked off from attack, while the party is trapped on a narrow conveyor belt. Fortunately, boxes will come trickling out onto the conveyor belt at a steady pace. Use the reaction command Fore to launch boxes into Oogie's lift, increasing the weight of it until it cracks under the pressure, sending Oogie falling onto the belt below. While he will occasionally toss bombs, maneuver lasers, or unleash a massive boxing glove on Sora, these can easily be avoided, and don't do a good amount of damage. Combo him while he's down and eventually, Oogie will once again become invincible as he runs back to his lift. After this sequence, the conveyor belt will suddenly be sped up considerably. Jump to avoid the spikes at the back of the belt, then continue launching boxes. After Oogie takes enough damage, he will begin to change up his strategy, moving his lift to different conveyor belts in order to avoid Sora. Pay attention to the colors of the panels at the end of the belts, as the corresponding colors will indicate which belt Sora will be taken to. Once Sora ends up in the same place as Oogie, the assault will continue, with Heartless pouring out onto the belt as well as boxes that can be launched. Get the Heartless out of the way quickly before continuing the effort to break Oogie's lift. Continue the tedious pattern until Oogie Boogie is finally defeated for good, earning Sora an extra item slot, Donald and Jack an increase in maximum HP, and Goofy the incredibly useful Once More. After the scene, reap your reward for clearing the world in the form of Magnet magic, then continue to Pride Lands.