Around 30km south of Phan Thiet, the spectacular Ke Ga lighthouse dates from the French era. Constructed in 1899, it sits on a rocky islet some 300m from the shore, towering almost 40m above the ocean. It's just possible to swim (or even wade) across if the tide is very low, but most visitors hire a boat operator (from 100,000d return) to get across.
You're no longer allowed to ascend the lighthouse.
Still, there are fine vistas from the islet east across the ocean and back towards inland hills. Note: guards routinely overcharge foreigners for entrance tickets.