Fast answer
A good Polignano beach day starts with the fact that the town's most famous swim, Lama Monachile, is also small, pebbly, crowded in peak periods, and sensitive to sea conditions. Use it as the iconic highlight, then keep Cala Paura, Ponte dei Lapilli, San Vito, Porto Contessa, and sea-cave boat context as separate choices. The right day depends on weather, sea state, access, crowding, services, and whether the group wants a swim, a view, or a boat-led cliff experience.
If you only do one thing
Default to a viewpoint and short-swim plan at Lama Monachile when conditions are good and crowd pressure is tolerable. If the group wants more space or a calmer coast rhythm, check San Vito or Porto Contessa; if the goal is cliffs and caves, check sea conditions and operator details before booking a boat.