Supplies for the Raft
Talk to Kairi once you gain control to get a list of things to collect -- two Logs, one Cloth, one Rope. Run along the beach and you'll spot one of the logs you need. Continue up the short steps near the waterfall and climb the ladder. Follow this path to the ship (which you may recall from earlier) to find the rope in the corner. Head back down the ladder and follow the ramp across from it up to a makeshift tree house where the cloth can be found.
The final item is found on the small island. To reach it enter the Seaside Shack, where you'll find a Save Point, and exit atop. Likewise, you can jump from near the waterfall to get up top and cross the bridge or swim behind the island and use the ladder. Return to Kairi with the items in hand to receive a Hi-Potion for your efforts. You can call it a day or explore this half of the island more if you wish.
If you talk to Wakka, Selphie, Tidus, or Riku they will offer up a duel. This is a good opportunity to get a little experience, gain Potions, or raise your score versus Riku.
He will throw his Blitzball at you and you can knock it back at him. Doing so results in 1 Tech Point and it leaves Wakka stunned. Use this opportunity to get in a combo. Wakka also does a jumping throw signaled by his shout, "Take this!" which can be knocked back for 2 Tech Points and a resulting stun.
She swings her jump rope around at you and you can deflect it back at her for 1 Tech Point and a stun. Like Wakka, the stun leaves her open for a chain attack. Selphie doesn't defend much so an all-out attack is a good option as well, especially since Selphie has fewer Hit Points than the rest of the gang.
He uses a jumping attack and often follows with a side step. Timing the parry on his normal attack will net 2 Tech Points but you can also swing just before he leaps to start a combo. Using Aerial attacks is a good idea against Tidus since it often leaves you behind him, especially after he uses a leaping strike.
After defeating all three Final Fantasy characters you'll receive the option to duel them in a 3 vs 1 by talking to Tidus. The best way to win this fight is to take out Selphie first since she has the lowest HP. It lets you focus on dodging Wakka's Blitzball while you smack her around.Take out Tidus next by countering his leaping strike, but watch for the chance to knock Blitzballs back at Wakka. Obviously, Wakka goes last and he has the most HP, but alone he isn't a threat.
Riku's tactic is to leap behind Sora and crack him in the back of the head. This should also be your tactic since Riku will parry your attacks from the front with ease. If you unleash a full combo on Riku he'll land on his back and spring forward with a vicious strike, just be sure to dodge it and you can counter.
Food for the Journey
The second half of the island opens up now. Head through the door that Kairi was blocking to enter the Cove and start a race with Riku. The best way to win the race is to simply run (don't jump) off the edge instead of following Riku. Run through the water and up the steps then run off the edge again. Run right of the boat and up the ramp, then jump up to the Star. Run off the edge and go back the same way you came. Riku takes his time jumping around which slows him down (and you too if you jump!) so running is fastest. You receive a Pretty Stone for winning the race.
With the race over, jump across the docks to the other side. To the right is a box that you can pick up and throw. To the left of the docks is an alcove high above. Position the box close enough so that you can jump up and claim the Protect Chain from the chest. Jump down and grab the Mushroom between the bushes. Jump down to the beach and push the rock out of the way to reach another alcove with a Mushroom . Talk to Kairi to get a list of items -- one Seagull Egg, three Mushrooms, two Coconuts, three Fish, and some Drinking Water.
Smack the trees nearby until you collect two Coconuts. Return to the Seashore and jump into the ocean to collect three Fish then talk to Wakka. Run under the waterfall to obtain some Drinking Water then head into the Secret Place. In the cave you'll find the final Mushroom and a scene occurs. Climb atop the Seaside Shack and up the skinny tree. Jump from here atop the other tree to grab the Seagull Egg. Return to Kairi to obtain another Hi-Potion and call it a day when you're ready.
The Door Has Opened...
After some cutscenes you'll gain control of Sora on Destiny Islands at night. The island is under attack by Shadows, but your wooden sword won't affect them! Head to the island where Riku is located for another scene. Sora obtains the Kingdom Key Keyblade at this point and can now damage the Heartless. You may defeat the Shadows as long as you would like but the objective is the Secret Place. Inside you'll find Kairi, but another scene takes over and you once again find yourself fighting Darkside.
BOSS - Darkside
This time around Darkside is a little bit stronger. It stills uses the Summon Shadow attack and Darkness Orb so you'll want to take advantage of the Summon Shadow attack to get the most damage in. In addition to these is a new attack, Darkness Rain. Darkside will reach into the ground and pull out a huge ball of darkness before using the attack. You'll be able to deal a significant amount of damage during this time, just don't get hit by the ball of darkness. Unlike the previous battle, you must win this one so don't hold back if you need to use a Potion.