The best way to explore Ayutthaya is by bicycle. The terrain is flat, distances between ruins are short, and cycling lets you stop wherever you like. Guest houses and shops near the train station rent bikes for 50 to 100 baht per day.
Tuk-tuks are everywhere and will take you between sites or do a full-day tour of the main temples. Agree on a price before you get in. A full-day tuk-tuk tour covering the major ruins typically runs 800 to 1,200 baht.
The historical island is compact enough to walk, but the heat makes cycling or a tuk-tuk more practical for covering the main sites. Some of the best ruins, like Wat Phra Ram and Wat Mahathat, sit close together in the central area.