The famous Phra Chinnarat Buddha image at Wat Phra Sri Rattana Mahathat
Phitsanulok wouldn't be at the top of anyone's "must see" lists of Thai destinations. There really isn't enough to see in the small provincial capital to make it worthy of being a destination in itself. However, if you're visiting Sukhothai, a stop in Phitsanulok may be interesting, or even necessary, if you're traveling by train or budget airline.
The central tower with the main west chapel on the right of Wat Phra Sri Rattana Mahathat
The most unique sights of Phitsanulok are its famous Buddha image foundry and the nearby folk museum run by the owner of the foundry. The other main sights of town are two Sukhothai era temples, Wat Phra Sri Rattana Mahathat and Wat Ratburana.
You can see the sights of the city in a day, easily, so you may not want to plan more than a night in the city. However, there are two big festivals held in Phitsanulok every year. A big temple fair is held in the grounds of Wat Phra Sri Rattana Mahathat every January, while the river running through town hosts very popular long boat races every October. A longer stay at these tmes may be in order.