Sarushima

The only natural island in Tokyo Bay, a 10-minute ferry from Yokosuka. Originally fortified as a Meiji-era military battery to defend Tokyo from naval attack, the island now has overgrown brick tunnels, crumbling gun emplacements, and forested paths that feel far removed from the surrounding urban sprawl. Summer adds barbecue areas and swimming beaches. The views from the hilltop take in the full sweep of Tokyo Bay, from the Yokohama skyline to the Boso Peninsula. Year-round access and a manageable size make it one of the more rewarding half-day excursions from central Tokyo.