I went to Sukhothai for the first time after having visited Bangkok and Ayutthaya. How nice it was, after the crowded city and the long bus journey, to explore a peaceful town, surrounded by pools and trees, where you find temple ruins that are several centuries old.
Sukhothai Historical Park, which became aUNESCO World Heritage Site in 1991, is home to a collection of remains in brick and laterite that you can visit on foot or by bike. By bike means you can visit the temples outside the park, especially those to the north and east of the town, which have very few visitors. You really must visit the park at night fall, the ruins are lit and the atmosphere is completely different, almost mystical....
Discover Thailand by visiting Sukhothai - it's the first step in discovering the incredible cultural riches of this wonderful country... a town that is not to be missed!
The kingdom of Sukothai only lasted a few years but its particular art and style marked its era. If you know the other capital, Ayutthaya, you can see the differences, a good reason to visit this other historic park.
Although you can visit it by car (which I think is wrong in principle), I always visit the site by bike, which you can rent from the village nearby, as the modern town of Sukhothai is about 10 kilometers south of the historic park. It's an ideal place for a family, the park has pretty woods and is well maintained and the Thais also like relaxing in Sukhothai.
The main site is inside an area marked by ancient moats and walls. I think it's a shame that they've divided the entrance tickets into zones and, yes, a basic ticket doesn't let you visit everything.
If I had to choose, I would say that, as well as the central square, it's the temples in the east, especially Wat Si Chum, which are worth the visit. Those in the north are mostly in ruins and a bit less impressive.