Mastihari Main Beach is a straightforward beach stop: sand, water, and a slower rhythm than the centre of Kos. It is the kind of place to add when the trip needs a swim, a picnic, or simply an hour of staring out rather than another formal sight.
Beach conditions can change quickly with season and weather, so check local notices before counting on swimming. Even when the water is not the point, the shore is still useful for a walk and a low-key afternoon.