I took a train out to Ayutthaya from Bangkok for the day and rented a bike to ride around the ruins. It was great and since I went later in the day there weren't so many people. Easy to get to and a definite highlight! Just take a train from Bangkok's train station heading to Ayutthaya (there are lots of attendants standing around waiting to help foreigners). When you get there, look around for bike rentals, but make sure you check out the bikes first because some are a bit rusty.