Instructions
Dig up fossils from star-marked areas on the ground. Three of these appear every day randomly around town. By digging on this hole, you either get a pitfall, gyroid or fossil. If it's a fossil, you can sell it at the store for between 2,000 to 5,000 bells.
Go fishing at any body of water. You must buy the fishing rod for 500 bells and swing its floater onto the water. Catch them until your pack is full then sell them at the store. A full pack of the best fish can net a possible 10,000 bells for you.
Plant non-native fruit around town. Your Animal Crossing town will begin with and only grow either an Apple, Orange, Peach, Cherry or Pear tree. The best strategy is cultivating a non-native fruit of your town by sending a letter to a villager with a fruit attached. In exchange they will give you a non-native fruit, which you must plant and cultivate until it grows into a tree with three fruits. Sell each fruit for 500 bells while planting more fruits to multiply your profit.
Buy White Turnips from Joan on Sunday between six to twelve o'clock. Each one she sells is worth about 100 bells, but prices will vary each time and she has an unlimited supply. Tom Nook buys turnips, and each day they either raise or lower in buying price. The best strategy is to sell them when its high by 30 bells more than you bought them for, or sell to break even. You only have until the following Saturday to sell, or else the White Turnips will spoil.
Hit money rocks with your shovel. Rocks will be scattered around town and if you hit one, they might give you money. Unfortunately when you hit a rock it will knock you back, but the trick is to go to the corner of the rock, dig a hole one square down and one square to the side, then hit the money rock as many times and as fast as possible. You earn 100 bells per hit.