Ayutthaya
Ayutthaya is a former capital of Siam, built after the fall of Sukhothai around 1350. Due to Ayutthaya's location it became a trade hub connecting China, India and the Malay peninsula. By 1700 Ayutthaya had grown to be the largest city in the world with traders drawn from all over the world.
Ayutthaya at that time was a beautiful city, widely acclaimed with golden palaces and grand ceremonies. However, in 1767 the city was razed following an invasion by the Burmese. It was a very effective invasion as little was left of the bustling metropolis. Today the only remains are some of the stone buildings of the time, which includes mainly temples and palaces. The invaders stole most of the gold and melted it down, so there is little remaining.
Ayutthaya is listed as a UNESCO World Heritage Site, which has resulted in many of the ruins being taken care of and now the city received many visitors. Given it's convenient proximity to Bangkok, cultures vultures would be well served with an overnight trip to Ayutthaya before heading south to the beaches.