The following Gummi Ship bonuses can be easily got if you do these achievements:
Berserk mode: reach Metal Level 30
Gummi Ship Level 2: complete Level 1 with "S", "A", or "B" ranks
Gummi Ship Level 3: complete Level 1 and 2 with "S", "A", or "B" ranks
Special Models: receive an "S" rank in the Gummi Ship missions
To get Synthesis Moogle bonus items, do the following task: you should give the following items to the Synthesis Moogle.
AP Boost: all Mythril Materials
AP Boost: five 25 types of Material
Defense Boost: all Bright Materials
Elixer: five one type of Material
Free Development: 5 types of Material
Free Development 2: 20 types of Material
High Drive Recovery: 10 types of Material
Megalizer: 15 types of Material
Serenity Shard: a total amount of 50 or more Materials
Serenity Shard 2: a total amount of 100 or more Materials
If you want to get the most powerful weapon, you have to defeat Sephiroth. He has fifteen health bars and is very strong so you will have to do your best when trying to defeat him. To fight him, go to Radiant Garden/Hollow Bastion at The Dark Depth area and defeat the MCP (Space Paranoids). Once you defeat him, you must talk to Cloud, then talk with Sephiroth again. After that, you will get Fenril, which as a 7 Atk and 1 Mag.
To revive your character, you have to perform the command "I won't give up" (for example, against the Shadow Stalker and Dark Thorn Bosses in Beast's Castle and Xaldin). Use Mickey because he is better than Sora and has stronger attacks. Anyway, you can use Sora instead.
To refill your health, you should do the following. When your level of health is low and no magic or potions, but a full Drive gauge, turn into any Form and you will get the desired effect.
To refill magic, in Cave Of The Dead: Entrance, clear out all the Heartless/Nobodies. Now you can use Magnet to make all the floating white balls explode. I found this way quite quick and effective!
To get keychains, you have to complete the indicated task:
Bond Of Flames: beat Axel at Twilight Town.
Circle Of Life: speak to Simba at the Pride Lands.
Photon Debugger: complete Space Paranoids.
Rumbling Rose: speak to Beast after returning to Beast's Castle.
Sleeping Lion: go to Ansem's study at Hollow Bastion.
Star Seeker: get Valor Form at The Mysterious Tower.
Fenrir: beat Sephiroth at Hollow Bastion.
Follow The Wind: beat Barbossa at Port Royal.
Monochrome: beat Pete at Timeless River.
Oathkeeper: complete Sora's first visit to Twilight Town.
Guardian Soul: beat Hades in the Underworld segment at Olympus Coliseum.
Gullwing: speak to Yuna after getting Guardian Soul.
Heros Crest: beat the Hydra at Olympus Coliseum.
Hidden Dragon: beat Shan-Yu at The Land of Dragons.
Holy Pumpkin: beat the Test Subject at Halloween Town.
Oblivion: beat Xigbar at The World that Never Was.
Sweet Memory: complete the Spooky Cave mini-game at 100 Acre Wood.
Ultima Weapon: get the Ultima Recipe in the Twilight Town mansion basement then make it at a Moogle Shop.
Wisher's Lamp: beat Jafar at Agrabah.
Wonders Of The Abyss: complete Chapter 4 in Atlantica.