First change your game's date to October 31. Fill your backpack with fruit and roam around town until someone comes to you asking for candy. Answer "No" and they will transform one of the fruits in your pack into a Jack-In-The-Box, which sell for a lot of Bells. Move so that the person that talked to you is off screen. Now walk back so that he talks to you again and repeat the process until all your fruit is gone.