Harmanite, the long sandy beach south of the Old Town, is the main beach of Sozopol. It is wide, gently shelving, family-friendly, and well-equipped: umbrellas and loungers for hire, beach bars, a paved promenade behind the dunes. In July and August it fills up; in May, June, and September it is half-empty and pleasant.
The northern end, closest to the Old Town, is the busiest. Walk south along the sand for ten minutes and you will find a quieter section.