This was Thailand's first capital. The old city of Sukhothai has about 45 sq km of ruins. If Wats are your thing then this is the place to visit classic Thai architecture.
Favourite spots:
Wat Si Chum
I think i visited almost everyone of the historical sites and ruins on a day that was blisteringly hot. I opted for a hired sawngthaew at a bargain price but cycling is another good option.
What's really great:
Wat Chang Lom
My driver was friendly and informative and took me to almost every ruin in the area. Some are in much better condition than others. The areas around the large ponds were very peaceful and there does not seem to be as many tourists as Ayuthaya.
Sights:
Wat Trapang Thong
The huge Buddha at Wat Si Chum.
Wat Mahathat and Wat Phra Phai Luang
Accommodations:
Wat Si Sawai
Within easy travelling distance from Phitsanulok.
Nightlife:
Wat Phra Phai Luang
I only made a day visit from Phitsanulok so no recommendations!