The Stalk Market is the most effective way to make a bundle of money. While you can make a lot of ACNH Bells, also you can lose a lot of Cheap Animal Crossing Bells
- notably, if you forget to sell the Turnips by Saturday when Nook's Cranny closes because on Sunday turnips gonna have rotten.
Other than that, essentially the most effective ways I've found is Rare Fruit orchards + Planting 10,000 Bells each day at the glowing spot (to Buy Nook Miles Tickets
if the tree is fully grown). Between the Money Tree and Rare Fruit Orchards, I'm in a position to get a day-to-day profit of 100,000 alone.
Floating Yellow Balloons are pretty reliable in dropping Bells. The highest amount I've ever got is 10,000 Bells - but I reliably get 5000 Bells at their store. Normally get at least a couple of these every day during gameplay to get a minimum of 10000 bells.