Check out the Old Ruins of Wat Kamphaeng Laeng and Wat Phai Lom
Not too far from Wat Mahathat Worawihan and the morning market, you will also find Wat Kamphaeng Laeng and Wat Phai Lom, the old remnants of the expansive influence of the Khmer empire that reigned over this part of Thailand in the 12th Century. Wat Kamphaeng Laeng especially is considered to be the oldest structure in Phetchaburi and although it is pretty small compared to other old Khmer ruins in Thailand, it is still worth a visit if you are interested in the history of this region.
Wat Kamphaeng Laeng is definitely the bigger one of the two but at Wat Phai Lom, you can still make out the beautiful handicraft patterns on its well-preserved facade, which is worth a look if you love history. Both temples are open 24/7 every day and there are no entrance fees for both foreigners or Thais to visit.