As the old capital of the Kingdom of Siam, Ayutthaya is filled with wats, wats, and more wats. Most all the sights are spread out on the island in the center of the city, which is essentially one large park, with the functioning modern town built around it.
My favorite site in Ayutthaya was Wat Mahathat and its Khmer architecture. The dozens of beheaded buddhas were decapitated by the Burmese back in the day, and now sit serenely between the banyans.
Catch the train from Bangkok's Hua Lam Phong train station for a very cheap and scenic ride.