Nha Trang is lined with an uninterrupted, fine stretch of sand: the Tran Phu Beach. This is the beach most used by both locals and tourists, with infrastructure such as changing rooms and bathrooms available throughout at a fee. Although the beach works great for recreation, it's far from being the area's cleanest and most attractive.
Alternatives worth exploring include the Bai Dai Beach, frequented by surfers,
Bai Tien Beach (stones rather than sand), beaches on nearby islands (such as Bai Tru on Hon Tre, and Hon Tam Island beach). The beautiful Diep Son Beach is known for the spectacular underwater sand path leading to the island (only seen in full during low tide).
If you're ready to travel further out, the Jungle Beach is a great option, but will take a couple of hours to reach. Surrounded by nature and fascinating flora and fauna, this beach is popular with both off-the-beaten-trackers and resort vacationers staying at the nearby Jungle Beach Resort.