The Sands of Tottori. Every prefecture has something nationally (and sometimes internationally) famous. In the case of Tottori, it's definitely the sand dunes (sakyu in Japanese).

It's on the beach facing the Sea of Japan. The dunes were formed by the ocean currents that deposited the sand on the coast for 100,000 years or so. I find that amazing since most of the beaches I know have the ocean currents eat away the sand.