As long as you reserve an entire day for Ayutthaya, there is no need to book a tour. You can simply take the train and ride with the locals. Once you arrive you can either hire a bike or a scooter and explore the different temples of the former capital of the Kingdom of Siam. There is a Buddha statue that has been completely integrated by a tree and there are some structures that are simply mind blowing.
We did the mistake to rent bikes on a hot day and sweated so much, that it took us almost as long to recover in a bar as we had been biking.