Just across the road from the ticket booth and entrance to the historical area, there are a couple of bike rental places. For thirty baht (just under a dollar) you can rent a bike all day. This is what all the guides I've read said to do, and they were right.
The ruins at Sukhothai are of a whole city, so they are scattered out over a fairly large area. It saved a lot of time to bicycle between the major clumps of ruins. Also, on such a hot day, bicycling means a bit of wind in your face, so it was nice to cool off a bit. There were areas to park the bikes near the major temples, and the rental place provided a bike lock. They also let me borrow a small bungee cord to fasten the bag I was carrying my shoes in to the back seat, since the bikes didn't have baskets.
After I explored the main section of the old city, I biked out a couple of kilometers to the northern section of the ruins. I explored several areas up there; the most important being Wat Si Chum, with its tall Buddha peeking out of a narrow door. The buddha's hands are painted gold; you can see the pictures below.