Jessica and I, along with two German girls we met, took the train from Bangkok north to Ayutthaya. The trip was about 2 hours and cost 15 baht - 30 cents. We rode 2nd class and my arse hurt a lot. There were mostly commuters and monks on the train, heading out to the suburbs and beyond.
Ayutthaya is one of Thailand's former capitals, from 1350 until it was sacked by the Burmese army in 1767 (wiki). Now, the new city has grown up around a collection of ruined temple sites. We rented old cruiser-style bicycles and rode to the famous ruins. Entrance to each was 30 baht (about 80 cents). We were able to climb up and inside of some of the incredible stone structures - very tomb raider.
After riding all over town and eating dim sum snacks at a dirty sketchy street restaurant, we made the standard trip to the city day market. There were some particularly nauseating areas with raw meats of fowl, fish, and other fare. There were several amulet vendors, and I bought 6 excellent carved stone Buddha necklaces.
The day ended with a wonderful longtail boat ride around the river that surrounds the city. Several of the travelers staying at the guesthouse went. We got some great views of backyards - many families were bathing, playing in the water, or just hanging out, and they waved and smiled kindly. Along the way, we stopped at several temples that are otherwise hard to reach by land.