Why visit
Mirissa is a beach town on the south coast, used on the Coastal Retreat and as the unwind chapter of a Signature journey. After forts and parks, it is where the itinerary exhales.
Beach and optional boats
Most days here should be unscheduled. Whale watching is seasonal and optional; it is never guaranteed, and the boat fee is separate. Surfing and simple beach time need no itinerary language at all.
With Galle
Galle Fort is close enough for a half-day without changing hotels. A chauffeur-guide makes that a civilised loop rather than a luggage exercise.
How many nights? Two is a minimum if you have been inland all week. Families often want three. See south versus east coast timing if you are choosing a season.