How do I get Silver and Gold Treats?

Often when villagers request any type of bug or fish, I give them a rare one because it gives better rewards. This often results in them yielding some bronze treats which are useful for raising a villager's happiness level by a few points.

On a loading screen, I noticed that there are silver and gold treats as well:

but I've never received these from doing any of my standard activities. How do you get the better versions of these snacks?

Silver treats be earned by completing the all daily goals. These are usually quite simple to complete, but as their name suggests they must be done before the day resets. You get three Silver treats for this.

You can only earn Gold treats by giving furniture or clothing to Gulliver to trade at Sunburst Island. Trading items marked as "Good" or "Great" increases your chances of getting them. Duplicate clothing and furniture from Fortune Cookies make for good fodder as their high value means the game usually marks them as Good or Great.

Note that the economics of this is not great - you usually get two or three treats, for furniture and clothing that can sell for a lot more. I already get more Bronze treats than I can use, so there's not a lot of incentive to get the higher level treats.