This was my first Asian WHS so it will have a little bit of a special place for me.
There are quite a few different ruins and temples to visit within the central area, each charging a separate admission fee. My favourite site was the first one I visited Wat Ratururana which had an impressive cluster of chedi's. The central one was particularly impressive as you could not only climb up it but also descend inside to see a small but impressively decorated chamber at its very heart.
I spent the best part of an afternoon dotting between the main sites, meeting fellow travelers and enjoying a relaxing stroll through the water lined park behind Wat Prah Mahathat. A week or so before I visited there had been pretty extensive flooding in the area and there were still a fair few places that were underwater. This meant I didn't get to visit any of the wats off of the central island. However it was very heartening to see that there had seemed to been a pretty good response from the local authorities to aid those affected by the flooding.
This was a really good day trip to make from Bangkok, and the sites seemed reasonably impressive.