Wat Phra Chetuphon Wimonmangkalaram or Wat Pho, as it's commonly known, was built in the 16th century and regarded as the royal temple of the reign of King Rama I, is famous for two things: (1) the 46-meter-long Reclining Buddha built in 1832 featuring the feet beautifully inlaid with pearls and (2) the Thai massage.
After wandering around wall paintings, you can stop by at the massage pavilion to try traditional Thai massage using ancient techniques to release muscle tensions.