Ayutthaya is an ancient temple complex, similar to Angkor Wat in Cambodia, although on a smaller scale. Situated just over an hour's drive from Bangkok, the site is easy to get to by car, taxi or you can take the train (at just 15 THB = $0.50 for third class wooden seats). When we visited, we used Tour with Tong, who provided a VIP van and Charlie, an amusing and engaging guide. There are also a number of trips on GetyourGuide that you can book more easily.
Designated a UNESCO World Heritage Site in 1991, Ayutthaya was founded in the 14th Century as the second city of Siam. Ideally situated in the middle of Asia, Ayutthaya grew rapidly after becoming a major trading centre and, by 1700, was the largest city in the world. With size came riches - European accounts tell of a grand city complete with golden palaces and a busy port. Today, little remains of the grandeur, after Burma invaded the city in mid 1700s, burning most buildings and stealing much of the gold.
However, Ayutthaya remains an interesting day out. Stone towers and monasteries provide a glimpse into how impressive the city must have been at the height of its success. Explore the Wat Phra Mahathat, taking in the rows of headless statues and see if you can find Buddha's head in a tree. Visit the huge reclining Buddha at Wat Lokayasutharam, not as pretty as that in Bangkok's Wat Pho but, at 37 metres long, just as impressive. One of our most vivid memories of our day (possibly because it was somewhat surreal) was the working temple with grounds full of models of fighting roosters, some as high as 2-3 metres.
Depending how long you have, there are numerous temples, museums and ruins to explore. Also of interest are the Portugese, Japanese and Dutch settlement areas, remnant of the city's trading past. Although these were largely destroyed by the Burmese, interesting museums give another flavour to the history of the city.
A point to remember: Ayutthaya consists of a number of temples and should be treated with respect. This includes wearing suitable clothes - no shorts, strap tops or anything too revealing.