Wooded Kingdom - Moons over the ? in the corner of the map
Solution 1:
You can jump off the top of the barrier/wall in the south east corner, right near the Odyssey, and reach the Deep Woods area. There are numerous power moons to obtain down there in addition to some surprises, and some plants that will allow you to get back up.
The clue that tells you about this doesn't seem to appear until you've beaten the game, after which one of the robots will appear there and inform you there's an area over the wall.