The Sukhothai Historical Park ruins are one of Thailand's impressive UNESCO World Heritage Sites.
The city's walls around the old city form a rectangle about 2 km long by 1.6 km wide, with a gate in the center of each side. This central zone covers the remains of the Royal Palace and some of the most important temples, the largest being Wat Mahathat. Outside the walled area you can find ruins of at least a 100 more temples in different states of preservation.
The park itself is large, but you can easily get around on a bike. Entry costs 100 baht pp and an extra 10 baht to bring a bicycle in. You can also rent a bicycle on the spot for 30 bath.
Apart from the major road there is very little traffic on the smaller roads around Old Sukhothai. It tends to get very hot during the day, so the fresh breeze on the bike is a plus.