The field north of Wat Phra Si Sanphet full of brick foundations, walls and little else was once the royal palace compound of the Ayuthaya kingdom. There's not much to see, but the Grand Palace in Bangkok was modelled on it, so that can help with your imagination. It makes for a nice, short bike ride. The entrance is in the southeast corner or you can walk in direct from Wat Phra Si Sanphet. It's open roughly 8am to 6pm.