I took the train from Bangkok to Ayutthaya then hired a tuk tuk driver just outside the station for 500 baht a day. It's no Angkor Wat, but it was a nice relaxing day trip as I was taken to various temples and the driver waited for me while I walked about the ruins. At some of the temples, there were very few tourists.

Wat Chaiwatthanaram ( map )

Wat Lokaya Sutha ( map )

Wat Yai Chai Mongkhol ( map)

Wat Mahathat ( map)

Wat Phra Si Sanphet ( map)

If you like ancient temples, also check out UNESCO World Heritage Site Sukhothai and nearby Si Satchanalai Historical Park north of Bangkok and south of Chiang Mai. Both have amazing temple ruins built during the Sukhothai Kingdom before the Kingdom of Ayutthaya.