The best option to visit the Son Tra mountain is to rent a motorbike from Da nang for a day (about 5$ USD) and drive by yourself around the peninsula.

Fansipan Mountain
From Hanoi, you can reach Sapa by train or bus. From there, you can easily reach Fansipan Mountain by motorbike,
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Really cool road! Bring a lunch and spend the whole day on the road of this mountain! Pretty impressive and huge Buddha statue !