Coins are an essential component found in Super Mario Odyssey. On top this fact, they aren't the easiest to come by, making them all the more valuable to obtain. However, there is a way to farm coins in the game, found in none other than Wooded Kingdom. By doing this trick, you can find and obtain roughly 1000 Coins in about ten or so minutes of playing.
In order to execute such a thing, head to the Observation Deck in Wooded Kingdom and use the Blue Button to activate platforms leading to the top of the tower. Once there, capture the Glydon Glide near the entrance of Secret Flower Field with the help of Cappy, and the land in the field below. In the northeast corner of the field is where you can capture a Coin Coffer to obtain more Coins; otherwise, grab a seed from the Steam Gardener at the northern end of the field.
After having the seed, head back to the Observation Deck, using the Blue Button's Platforms to reach the top once more. Jump to the highest platform after you, planting the seed to then ride a stalk high into the sky. This will bring you to a challenge area that holds roughly 250 Coins if you can collect them all without falling back to the Observation Deck. This same process can be repeated to, aforementioned, obtain about 1000 Coins (if all collected) in about ten or so minutes. Good luck!