Three days is the most common budget for Nha Trang. It is enough for one island day and a handful of city sights without turning the trip into a race.
Day 1 — Get your bearings. Check in, eat bun ca for lunch, then give the afternoon to the centre: Po Nagar Cham Towers, then Hon Chong once the sun has softened. Late afternoon, come back for a swim at Tran Phu Beach. Walk the seafront in the evening and eat nem nuong.
Day 2 — Go to the islands. Give this a full day. Boat tours from Cau Da port usually pair Hon Mun for snorkelling with Hon Tam or Hon Mot for lunch and lounging. If a group boat doesn’t appeal, charter privately or take the cable car to Hon Tre instead. Back on shore by late afternoon, with the evening for seafood.
Day 3 — Slow down and leave. Pick a direction for the morning: Long Son Pagoda and the Stone Church if you lean towards heritage, or the Institute of Oceanography if you have children along. Stop at Dam Market for dried goods before heading to the airport.
On pacing. Cam Ranh airport is about 35 km from the centre, so allow roughly an hour for the transfer and don’t schedule tightly against your flight. Between 11am and 2pm the sun is punishing — that is the window for eating, resting, or being indoors.