Hòn Tre is the large island facing Nha Trang across the bay, visible from much of the waterfront. It is heavily developed in places, but the island still matters as one of the city's main day-trip and resort landscapes, with beaches, viewpoints, and boat approaches.
The visit is most appealing if you want a beach-and-activity day rather than another urban sight. Check access and ticketing before going, since much of the island experience is shaped by resort and amusement-park operations.