I have rented a tuk tuk. If you browse Ayuttaya, the town itself, then venture by the river, where the evening market it, there is a nice athmosphere and a nice view on the temple on the other side of the river!

Then for exploring the old city, one day is more then enough. I rented a Tuk-tuk, who showed me around. With a guide, you will learn more about the place, but then you have to be interested in details of ancient history!!