Wat Mahathat houses the Phrathat Nakhon, the second-most important that (pagoda) in the province. In addition to the elegant spire, there's a small museum here that may be of interest. It mainly houses a collection of Buddhist and Hindu images.
The tower was built in 607 AD on the orders of a army chief from Vientiane who brought his troops here to settle. The information sign in front of the temple seems to say he was a deserter, but I think it is mistaken, and there's nothing in the Thai version of the information to suggest that. The spire was last fully renovated in 1922. The museum is in a small building to the left of the main chapel as you face it with your back to the entry gate.