Khamphaeng Phet doesn't receive a lot of visitors, I think mostly because of it's proximity to Sukhothai. However, because of the fact that Sukhothai is only 1.5 hours away, it is worth visiting as a day trip along with Si Satchanalai (in the other direction from Sukhothai).
There are a few temples in the city, along with the National Museum (pretty much every province has a national museum).
You are supposed to pay to view the ruins (I did), but it is open, and no one seems to really care.
Outside of the walled section of the city, there are more temples in a hilly, forested area. There are apths, and it behooves you to rent a bicycle... or a car, maybe.
The temples don't have the restored look that the ones in Sukhothai and Ayathaya do, and they have a natural feel about them.
Like I said, they aren't visited as often, and so you'll often wander around with the place to yourself.