During my first visit, despite the gloomy skies, I still went ahead with the seven hour bus ride from Bangkok to Sukhothai just to visit its famed temples. I had learned upon my arrival that much of Sukhothai was submerged under water just a few days before. In fact, there were sandbags all over the place and you could see that the water level of the rivers was high. On the way to the park, it was still raining. But when I arrived, the rain suddenly stopped and the sun came out later in the day. So I was lucky!
While it's not difficult to walk around the park, you can rent a bike or hire a motorbike or tuktuk to speed up your pace visiting the ruins scattered around the park. I hired a motorbike with driver from my hostel to take me around.
Last year, I was back again in Sukhothai for a workshop of the ICOMOS International Cultural Tourism Committee. This time, the weather was beautiful and the sun was out! Wat Mahathat was the largest and principal temple of Sukhothai. The wat is surrounded by a wall that is said to represent the outer wall of the universe. Around the wall is a moat representing the cosmic ocean. The large Buddha images and chedi spires in the lotus-bud motif are iconic images which adorn many photos from Sukhothai. If you look closely, you will see that some of the original bas relief details layared with stucco can still be seen at Wat Mahathat.
There are so many ruins and historical sites in the park. And depending on your level of interest (for some, if you've seen one, you've seen them all), you can try to visit all the major ones, about twenty or so, or select the more interesting sites if you want to take it slow.
Another famous temple is Wat Si Sawai which features three Khmer-style towers. It reminds me of Angkor Wat, and was in fact, built by the Khmers as a Hindu temple. Wat Sa Si is a temple that is built on an island in the middle of Traphang Trakuan Pond.
Then there is Wat Si Chum which features an eleven-meter Buddha image covered in stucco seated inside a brick Mandapa building. This 13th century temple is also known as the Temple of the Bodhi Tree. I had a photo taken right beside the Buddha's large hand that has been covered in gold leaf offered by devotees.
I can't remember how many temples I got to visit. I tried to visit them all since it's a single ticket for all the temples within the inner city, and another ticket for all those outside. Hopefully you don't get all templed-out before seeing them all because each Sukhothai temple is definitely worth a visit.