What is a a picnic spot?
"I wasn’t sure what I’d expected to find but everything looked just the way I remembered. The quarry was both beautiful and intimidating, like a picnic spot on the moon. You could see where the miners and their machines had sliced through the rock, but trees and grass and vines were reclaiming any area with soil and even sprouting through cracks in the granite"
Is it a simple picnic spot or is it some metaphor or maybe a crater?

Question

Answer

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? :)

Answer

"... 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."  :)

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