Nakhon Si Thammarat National Museum is the best indoor stop for understanding why this city matters. Its collections cover archaeology and religious art from Nakhon Si Thammarat and the neighbouring southern provinces, with material that points to the region's long Indian Ocean connections.
The museum is especially useful before or after Wat Phra Mahathat. It gives shape to the older kingdoms, temple art, and local styles that are easy to miss when you only walk the streets.