This is a metaphor. A picnic spot is simply a pleasant place/location/spot for eating a meal outdoors. A quarry is a place where rock or other materials are dug or cut from the earth. Abandoned quarries often have steep sides of exposed rock and partially level bottoms where water collects to form ponds. This passage seems to describe an old granite quarry. The rock walls and floor are still scarred from mining activity, but plants are growing from exposed patches of soil and through cracks in the rock. "Like a picnic spot on the moon" is a metaphor describing the contrast between the charm and beauty of the plant life ("like a picnic spot") and the stark, scarred rock walls ("on the moon"). Does that help? :)

"... intimidating, like a picnic spot on the moon." would be a similie, I think. Technically not a metaphor as it uses "like a". It is a similie. "... intimidating, a picnic spot on the moon." would be a metaphor, I think. or "... intimidating, a wonderful picnic spot on the moon." :)