From Phitsanulok, take a bus directly to Sukhothai old city....
We really only stopped here because we got on a train from Ayutthaya to Chiang Mai that had no air, just windows and people walking up and down the aisles selling hot food the ENTIRE time. Still being incredibly jetlagged, we needed a break from the train. We got off at Phitsanulok and quickly realized that we were the ONLY foreigners. Nobody could help us, there was no english written anywhere, and we couldn't find a guest house. After one sweet young boy trying to help us, we finally stumbled upon maybe the only guest house there. The next morning before boarding the train to continue heading north, we walked around the morning market which I am still impressed by when I think back on it. Phitsanulok was probably the least-touristy spot I have ever been to in my life, and I'm so happy that we stopped there.