I earned the Platinum Trophy for the game within six days, and the in-game 100% with it. I had done the majority of the things to do before I finished the game; post-final boss stuff included but was not limited to the extra boss(es, if you include the Gummi Ship stuff), grinding to 99 which was surprisingly quick, earning scores for the Classic Kingdom minigames, completing the Records section of the Gummiphone, and I believe the last thing I did was to mow down 200 pirate ships, which I largely took care of during my initial visit and went by faster than I'd anticipated.
I haven't technically 100% completed the game, as I'm missing a few Gummi Blocks, but I've earned all the blueprints and completed the checklists and such, so it at least says I've 100% completed the Gummi Ship. Definitely the easiest 100% completion of a KH game, except for maybe 0.2 if you really want to count that.