Ten sounds from outdoors: rain, thunder, the sea, fire, dry leaves. They are the hardest in the whole game, because they have no clear shape the way a dog's bark does; you have to listen to the whole texture. A good category once the child has mastered the others.
Rain, thunder, wind, sea, river, fire, birds, cricket, waterfall and leaves.
Because several sounds resemble each other: rain, river and waterfall are all water. Telling them apart means paying attention to rhythm and intensity, not just to the kind of sound.