When you mail letters to your neighbors in your town, attach a present! Put maybe shells you find on the beach or your town's native fruit. Wait for 1-2 days until you get a reply. When you do get mail, check to see if there are any "presents" attached to the letter. If there is, take the present out and open it! You may get exotic fruit, furniture, etc. If you don't like what you got, sell it. You'll get bells for free. (Remember that you didn't spend anything on the Fruit or Shells)
If it is exotic fruit that is not native to your area, dig it in the ground with your shovel and wait for it to grow into a tree. When you see the fruit coming, shake it off the tree and sell the fruit to tom Nook. You'll get 500 bells for each fruit!