Ayutthaya is about one and a half hour from Bangkok. Ever since I was a little girl, my family has regularly travelled to Ayutthaya to pray and bring presents to the monks of Wat Na Phra Men.
I also enjoy to walk around the magnificient ruins of Wat Chai Watthanaram - especially "off season" when the site is void of people. Going there on a quite day is very fascinating - it is as if you can feel the presence of pure consciousness and spirituality surrounding the ancient ruins.

You can read more about the temples and how to get there at http://wikitravel.org/en/Ayutthaya