What about seashells? Even though I'm sure I've never had a town close to perfect, I've always put as many seashells as I could fit on my bridges and ramps down to the beach.
Pretty much anything that spawns in your town naturally isn't trash. [...] clovers spawn naturally.
Are you sure about that? For the past few weeks my villagers have been complaining about the cleanliness of my town. Understandable, since I had to dump my entire closet onto the beach because of storage needs for turnips. Now, however, I've picked everything up - except for a few 4-leaf clovers, and they're still complaining. Does it take a few days for the villagers to notice that I cleaned everything up, or do clovers indeed count as trash?