Black Rock Sands is the broad beach escape near Porthmadog, with firm sand, low-tide rock pools, dune habitat, and big views back toward the mountains. Despite the name, the main impression is openness rather than dark rock.
It is best in decent weather, but it is not just a sunbathing beach. The Wales Coast Path approach from Portmeirion and Borth-y-Gest makes it part of a very good walking day.