Wat Chedi Yai is a small temple ruin on the southwest side of Ayutthaya's city island, inside Somdet Phra Sri Nakharin Park. It is not one of the headline monuments, but the large chedi is easy to pick out from the road and makes a useful quieter stop if you are already exploring this side of the old city.
The site is best treated as a short pause between larger ruins rather than a destination on its own. That can be a virtue in Ayutthaya, where the famous temples attract most of the traffic; here the mood is simpler, greener, and more local.