Where are the densest locations for collecting coins?

I'm saving up for a 9999 coin item in the shop (post-game content) so I need a lot of coins. Outside of the few vault chambers associated with a few power moons, are there any dense locations for farming coins?

Solution 1:

With the release of v1.2.0, Luigi’s Balloon World is now the best way to collect coins.

Luigi’s Balloon World is a minigame added in an update on February 21, 2018 and is unlocked after defeating Bowser.

Talk to Luigi on any stage to view a list of balloons hidden by other players. You can earn money multiple ways:

• Finding balloons hidden by other players.
• Having other players fail to find a balloon you hid
• Getting a streak by finding multiple balloons without failing
• Ranking up by completing challenges

I can’t say for sure whether this method is faster than the bonus area in Bowser’s Castle, but it is certainly a more fun and less repetitive way to earn money,

Solution 2:

Looks like the growing consensus is that the bonus (beanstalk) areas are the densest.

Arekkz recommends the bonus area in Bowser's Kingdom. Turn around from the "Beneath the Keep" checkpoint, follow the roof on your right towards the column of gold rings, and the seed/planter is on that side of the lower level. Estimated 150–175 coins, and you'll spawn so you can easily run it again.

As an alternative, IGN and AbdullahSmash recommend a loop through the Wooded Kingdom's Observatory Deck bonus area. Starting from that checkpoint, grabbing the purple coins, heading towards the seed dispenser, coin purse in the flowers below, and returning it to the Observation Deck, IGN estimates you can make 1000 coins in eight minutes.

In any event, the bonus levels are definitely faster than pulling coins in the slot machine or other schemes that involve getting 30 coins and repeatedly respawning. But here's hoping that there's an automated, infinite coin location hidden somewhere.