Give a lot of gifts to one character and complete that game. Start a new game on that saved game file and choose to play as the character that received the gifts in the previous game. Those items will be available in your inventory in the new game.
Successfully complete the game and save. Resume the game with the same saved game file to have two additional songs, "How Crazy Are You?" by Meja and "Is This Love?" by Bob Marley selectable under the Radio Station.
Swimsuits in exhibition mode
Earn or buy swimsuits in story mode (Zack's Island). The swimsuits that you acquire in that mode will also be available in exhibition mode.
Enter the options menu, then change Zack's language to Japanese.
Successfully complete the game. When you start a new game, you can immediately advance to the end. Select the "Leave Tomorrow" option at the hotel or pool menu to skip to the ending.