Aluminum Christmas Trees: Sea-salt ice cream. A lot of people in North America and Europe thought that it wouldn't possibly exist because the salt would lower the freezing point of the water, preventing the ice cream from solidifying. Or just that nobody would think salty ice cream would be good anyway. Actually, it's sold at Tokyo Disneysea - and has been before Kingdom Hearts II.
Author Appeal: The reason it was worked in. It was also worked in because he imagined what the characters would eat over summer vacation.
Executive Meddling: According to Tetsuya Nomura, he had originally planned on only including characters from Final Fantasy titles he personally designed. He eventually caved in to the urging of the staff, which is how Setzer and Vivi were added to the game. The gamble perhaps didn't pay off as well as they would've hoped; Setzer, in particular, was criticized for heavily deviating from his original incarnation's personality, moreso than was par for the course in KH.
Chip Cheezum and friends popularized "Dogstreet" for Pence, named after the writing on his shirt.
Fandom Nod: In addition to all the Disney/Final Fantasy references, there is a literal fandom nod during Cloud and Leon's snark-fest in the sequel. As they do their Back-to-Back Badasses moment, it echoes the rivalry between fans of their respective games.