The most important temple of the province Nakhon Si Thammarat is clearly Wat Mahathat, located in the southern part of the town. It is the oldest temple of the whole region, in fact it is the temple chronicle which provides the early history of the kingdom of Nakhon Si Thammarat.
The full name of the temple is Wat Phra Mahathat Woromaha Viharn (วัดพระมหาธาตุวรมหาวิหาร), and the actual age is not really known. Two versions of the temple chronicle exist, however centuries of copying and amending the text turned the oldest parts into something hardly distinguishable from legend. The reason to build a chedi at that site was to enshrine a tooth relic of Buddha, which according to one chronicle happened together with the foundation of the town in the year 1176. Yet both chronicles agree on several reconstructions of the chedi after it fell into disrepair, so the modern 74m high chedi in Sri Lankan style isn't the original one for sure.
The main chedi is surrounded by 173 smaller chedis in the courtyard, and a hall encircling it with many Buddha statues facing in all four directions. On one side of the chedi is the staircase to climb onto a gallery. The hall with this staircase alone is worth seeing, with figures of giants and lions protecting it. On the gallery one can encircle the chedi completely, and the sound of hundreds of small bells moved by the wind give it a great atmosphere.
Also within the temple area is a temple museum, which shows many historical items from the town. As it is already four years ago I can hardly remember any details on the exhibition, except that it seemed to me a bit unsorted - but still definitely worth visiting. South of the temple is a market, which among many other things also sells souvenirs styled after the traditional Lakhon figures used in shadow plays. However as I was traveling with Thai we only bought some snack there.
The town has several more historical places I haven't had the time to visit then - doing it as a day trip from Surat Thani only left some hours, and we spend most of the time in Wat Mahathat. I could at least take a few photos of the city wall, the city pillar shrine and Wat Yak - however though we passed them I missed the two Brahman shrines Hor Phra Isuan and Hor Phra Narai, and most of all the local branch of the National Museum would have been worth a visit.
an English book on the history of this town, the visit would have been even more worthwhile if I had read it before and already knew about the significance of the various sites. I just wish it'd have more such books on other regions of Thailand.