Nearly 20 days late, but you can return to previously visited minigames to grind for items upon reaching endgame; they can either be used to cast stronger spells, which I think do extra damage (up to 3 health bars) depending on which attack you use them on, or to nullify/reflect certain types of attacks (the gems). Also, you can go back to Puck after revisiting Southern Ruby Island to get access to the strongest spell in the game.