Sukhothai is the location of Thailand's greatest ancient temple sites. There is the "new" city, which is a typical provincial capitol, and there is the "old" city which is located about 14 km down the road. It is very accessible, and there are frequent buses which run along the road. Like most places in Thailand, people know where you are going (because you look like a tourist), and will steer you towards the park. The temple ruins are similar, but older that the ones in Ayathaya, and it is worth visiting both historic parks.
Sukhothai actually has an airport. Bangkok Air built their own airport when they were developing themselves as a tourism-focused "boutique" airline. They seem more mainstream now, but they built the cutest little airport you have ever seen - I thought it was a hotel the first time I drove up to it. Bangkok air also owns airports in Trat (for Ko Chang) and Ko Samui.
... and eventually, the owners made some renovations that were probably overdue anyway. It's a cool little place now, and naturally, the price went up a little bit. The last time I stayed there it was 500 baht ($15), which was twice the price as the first time I stayed there.
Of course there is the park itself. The park is quite large, but you can probably concentrate on the central "square" and not worry about the outlying areas. It is not expensive to get in (40 baht or so), and you can rent a bicycle to travel around in. It makes for a nice day, and you could easily spend a second day if you are really into it.
Overall, Sukhothai Historical Park was impressive. Some people might think that this is a bit to far off the beaten path for them, but I would say that the park is comparable in quality to Angkor Wat in Cambodia (albeit, smaller). Sukhothai is much easier to get to, and far less expensive, and I think that anyone willing to spend the money to go to Angkor Wat should also spend a couple days in Sukhothai.