We would give Da Nang a week in March and do one thing properly each day. The long wet season on Vietnam's central coast has mostly passed and the summer heat is still some weeks off, so with highs around 27°C the city is comfortable on foot for most of the day. Rain still falls on roughly ten days and the sea has not quite settled, which rules March out for anyone who wants a week flat on the sand. For sightseeing on foot and long lunches, it is hard to better.
Start with the Museum of Cham Sculpture, by the western end of the Dragon Bridge. Founded under the French to gather the carvings of the Champa kingdom, which ruled this coast for centuries, it holds one of the largest collections of Cham art anywhere. The rooms are named for the places the carvings came from, My Son, Tra Kieu, Dong Duong and Thap Mam, so a slow circuit doubles as a map of the old kingdom. Give it the whole morning. Come back to the river on a Friday, Saturday or Sunday evening, when the dragon's head breathes fire and then water from around nine; stand well back unless you want the spray.
South of the city, the Marble Mountains rise abruptly from the flat coastal plain. Thuy Son, the one most people climb, is threaded with stone steps and cave shrines, and in Huyen Khong cave daylight falls through an opening in the rock onto incense smoke and altars. There is a lift for part of the way, but the steps suit the place better. The Quan The Am festival is held here over three days of the second lunar month, with boat races and lanterns after dark. The lunar calendar puts it in March in some years and April in others, so look up the dates for yours. At the northern end of the bay, the great white Lady Buddha at Linh Ung Pagoda looks out to sea from the slopes of the Son Tra peninsula, and the forest behind is home to red-shanked douc langurs, best seen in the early morning. Watch them from the road and never feed them.
Between excursions, eat mì Quảng, the region's own noodle dish: flat rice noodles in a shallow pool of turmeric-tinted broth, topped with pork or shrimp, herbs, peanuts and a toasted rice cracker to crumble over. Han Market's food stalls serve it, and Mỳ Quảng Cô Sáu is listed in the MICHELIN Guide for its version.
The sea off My Khe beach can still be changeable early in the month, and the water tends to be calmest from April, so check the flags before you swim. The beach is at its best around dawn, when residents come down to swim and exercise before work and the sun rises out of the sea in front of them.