Wat Hat Yai Nai, Hat Yai
This is home to a large reclining Buddha, the 3rd largest in Thailand apparently, and a small mausoleum with a souvenir shop in the base.
It is a little way out of town so you need to use a local taxi or tuk-tuk to get there if you don't have your own transport. The cost of getting there should only be 10 - 20 Baht so don't listen to Tuk-Tuk drivers around the station who ask for 100 - 150 Baht. They are just trying to rip you off. Walk to an area away from the station where these pirates prey on farangs and find an honest driver.