Sasebo is a working harbour city on the west coast of Kyushu, shaped by shipyards, the Japanese navy, and a long American base presence. It is less polished than Nagasaki, but it has a real port-town character: steep streets, sea views, jazz bars, burger shops, and ferries moving between the islands.
The reason to come is the coast. Kujukushima, the "99 islands" scattered off Sasebo, is one of western Kyushu's best seascapes. Cruise boats leave from Pearl Sea Resort, while viewpoints such as Tenkaiho show the islands as a pattern of green humps and blue channels.
Sasebo also has Japan's oddest Dutch day out: Huis Ten Bosch, a full-scale Netherlands-themed park with canals, brick facades, gardens, and seasonal illuminations. It is artificial, but on its own terms it is impressively built. Combine that with a harbour walk and a Sasebo burger and the city easily justifies a spare day.