Karimunjawa is where Javanese families go when they want a beach and no crowds. Only five of the twenty-seven islands are inhabited, and the whole group is a marine national park.
The snorkelling is the point: shallow reef in bath-warm water, reachable by a short boat hop from the main island.
There is no rush here and very little to do, which is the appeal. Most itineraries put it at the end, after the volcanoes.