Bug Harvesting at the Tortimer Island is the best way to earn bells (in my opinion). But to make the most out of it, what I do is sell it to campers rather than the retail shop. Campers would usually ask you to play a game called "What Do I Want to Eat?" and if you win, you'll get more bells for that item. (although it's a little time consuming)
First, they would tell you what the color of the fruit they want to eat; it's either reddish or not red.
If they say reddish, he/she will give four choices: Cherry, Peach, Strawberry, Grape. I suggest you pick Strawberry first. If you got it correct, they'll buy the item/bug right away. If it was not the right fruit, he'll give you another clue. He/she would mention the size/shape of the fruit, if it was bite-size choose cherry; if it is round choose peach.
If the first clue is not red, the camper will give you the following choices: Apple, Banana, Orange, Pear. Pick Orange first. If it was the correct answer, they'll buy the item/bug right away. If it was not the right fruit, he'll give you another clue. He /she would mention the shape of the fruit. If it was long pick banana; if it was round pick Pear.
I usually sell a golden stag at around 50,000 bells each from this game.