Sukhothai is the place for bicycling, it started already with bicycling in the old city of Sukhothai. But there is so much more to see between the old and the new city of Sukhothai. It is a beautiful upcountry area with rice-fields, small villages, small factories, water buffalo's and just the way of life of Sukhothai. So the historical park and the upcountry area make a perfect mix for a 1 day bicycle tour (or if you are lazy a half day). I would recommend to do a full day. For that you should stay at least 2 nights in Sukhothai. There are many nice guesthouses and resorts in Sukhothai.
1. Cycling Sukhothai
There are a few people that started to organize bicycle tours and 2 of them I know personally. First there is Cycling Sukhothai by Mem & Ronny". Ronny is a long-term resident of Sukhothai who married many moons ago with Mem. While being the manager of a guesthouse he started to do bicycle trips for himself. But slowly guests started to join and the first tours were born. Now he has together with his wife a small tour company. They have about 25 mountain bikes and a few hook-on bicycles for children. And of course children seats. They do half day and full day tours, as a combination of the country side and the historical park. Mem speaks very good English and Ronny speaks French, Dutch, English and of course Thai.
2. Sukhothai Bicycle Tour
Then there is Sukhothai Bicycle Tour by Jib & Miouw, who both come from the area of Sukhothai. They both speak good English and have been working in the tourism sector for more as 15 years. They also have about 25 bicycles. Of course they also have hook-on bicycles for kids, etc. And of course they do like-wise tours around Sukhothai.