After collecting the impressive mark of 999 Power Moons, the Odyssey Airship will finally be completed filled. Once it is, a giant Top Hap will adorn Princess Peach's Castle in Mushroom Kingdom. After it has, take the climb up the castle and reach the top of the black hat. Once you have, go in the center of the top of the hat and spin Cappy. By doing so, you will unlock a special Easter Egg Endscene in appreciation of all your hard work.