Hòn Tằm sits just offshore from Nha Trang, close enough for a half-day island break but far enough to feel removed from the city beach strip. The island is best known for calm water, sand, and organized beach facilities, so it works well if you want a softer island day rather than a rough boat-and-snorkel expedition.
This is a managed resort island, not an untouched hideaway. That has advantages: boats, loungers, food, showers, and mud-bath-style add-ons are easy to arrange. Go for swimming and views back toward the bay, and choose another island if you are mainly chasing coral or solitude.