I would rather spend my vacation in a big city than in a mountain cabin.
Parallel structure refers to having the same structure or parts of speech in the same sequence and function. In the sentence, the parallel structure that needs to be paired with is “in a big city” which is a prepositional phrase; so, the choice that has the same format is “in a mountain cabin” which is also a prepositional phrase. “To go,” “staying,” and “being” are obviously different for they are either an infinitive phrase or participial phrase.