The ruins of an ancient Buddhist temple is pretty impressive, especially on a good weather day. You can rent ancient Thai costumes and take some really nice photos here.
The holiest temple of old Ayutthaya, destroyed by Burmese in 1767. The main attractions are the Stupa/Chedi (where kings are buried) and remains of the old holy city.
A special local dessert: Roti Sai Mai (colourful candy floss wrapped with rice flour skin). First time trying it and love it. It is a very interesting kind of dessert.