I went to Pefkohori at the beginning of May. The place was not overcrowded but still it had a wonderful athmosphere because of its colorful little shops and beach taverns.
The waters are crystal clear and there is a magnificent view to Sithonia from the beach. There are good beach facilities as well as many places for entertainment. Daily boat cruises are offered as well.
If you get bored you can always take your car and go to one of the many nearby towns.
I will definitely go there again and recommend the town to people looking for good time and fun.