Sukhothai, founded in 1238, is about 267 miles north of Bangkok and was the first capital of Thailand. Its position as capital lasted for around 120 years before the rise of Ayuthaya. In the 13th & 14th centuries Sukhothai was a rich city and served as the cultural, as well as administrative, heart of the nation. The architecture of the period is deemed to be the classic Thai style and the magnificent temple ruins bear witness to this. The ruins themselves have been made into an historical park and cover an area of 30 square miles. If you have time a great way to see the ruins is to hire a bike and slowly cycle around the park.
If you are staying in Sukhothai the nearby Si Satchanalai-Chaliang Historical Park is also worth a visit. Here the ruins are not quite so well kept and the rustic feels adds to the atmosphere.