There are several ways to get to Ayuthaya: first, there are organized bus tours that stop at Bang Pa In, second, there are two day river tours with a elegant ships  (e.g. the Mekhala) that are very nice if you have the time. Also one-day (you really need a full day), but more fun than the bus is going on your own by train. The trains leave from Hualompong station in central Bangkok several times per day (choose an early morning train) and need two hours to get to Ayuthaya (don`t forget to ask at the information counter for  a schedule with all the trains to and from Ayuthaya, as you might like to stop on the way in Bang Pa In). Get a seat at the window and once the train gets past the airport, watch out for little villages, farmers ploughing ricefields, lotos ponds, and other interesting things.

Ayuthaya has been built on an island in the Chao Phraya river, with canals running through the city (like Bangkok or rather, Bangkok was modelled after Ayuthaya). Modern Ayuthaya is an unspectacular Thai provincial town; the interesting historical sites are all located on the western side of the island. Take a Tuk-Tuk at the train station (for about 250 Baht per hour); the drivers know the important places.

Start your tour of Ayuthaya at Wat Raj Burana. The elegant ornaments on the central Prang and the ruins of the buildings nearby let you catch a glimpse of the splendour of old Ayuthaya.  In the year 1958 a crypt with precious jewellery and Buddha images was discovered at Wat Raj Burana, that can now be admired in the Chao Sam Phraya Museum (a visit is recommended, but it best fits in after the next stop, Wat Mahathat ("Temple of the Great Relic"), by many considered to be  the most beautiful temple of Ayuthaya. Wat Mahathat  has a 50 meter high Prang, partly destroyed, that is  surrounded by smaller Chedis and impressive Buddha images. In the building nearby you can see a model of Ayuthaya as it must have looked like 300 years ago.
 

The next noteworthy temple is Wat Phra Ram, but on the way you should stop at the Chao Sam Phraya Museum and look at the exhibits from the crypt at Wat Raj Burana. Wat Phra Ram was built near a large crescent-shaped pond that reflects the lovely old ruins. Wat Phra Ram is one of the oldest temples of Ayuthaya; it was built in 1369 by the son of King U Thong, the founder of the city. You enter the temple complex through elephant gates in the old stone walls. The main terrace is dominated by a very elegant high  Prang and adorned with lots of depictions of mythical animals and Buddha statues. The temple complex is almost enclosed by a large, u-shaped pond that reflects the ancient buildings and towers - a beautiful sight!

Two other nearby temples are also worth a visit - both Wat Phra Sri Samphet and Viharn Phra Mongkol Bophit are located only about 400 meters west of Wat Raj Burana. Among the ruins in Wat Phra Sri Samphet three high chedis stand out - they have been restored  (under them the ashes of three kings are buried) and give you an impression of the beauty Ayuthaya must once have had. Viharn Phra Mongkol Bophit, strictly speaking, is not a historical site (the temple was built only 30 years ago), but it houses Thailand`s largest Sitting Buddha statue  that was made at the beginning of the 17th century in Ayuthaya. The enormous bronze statue had for centuries  been exposed to wind and rain (and before that to the attacks of the Burmese) until it was restored in 1958 and placed in the newly built Viharn. This statue is highly revered by the Thai and lots of pilgrims come to pay their respects to it - and also lots of tourists, which is why you will find here a large parking lot as well as many souvenir shops!
 

On the way back (only a few kilometers south of Ayuthaya) you can stop in Bang Pa In with the old Summer Palace of the Kings of Ayuthaya (later rebuilt by King Chulalongkorn). The station sign is in latinized transcript as well as in Thai, and the palace is only a short walk from the Bang Pa In train station. The palace has been built on a little island in the river and is very pretty. Most attractive is probably the Water Pavillion. This graceful building with its four-tiered roof that rests on elegant columns is a masterpiece of Thai architecture. There is also a little park and a building in Chinese style that can be visited.