Road Station. Beside 水無川 on Route 251. Roughly 4 km south of the port and walking distance from the Memorial Hall. At first glance, this may just appear to be an ordinary road rest station for long distance drivers (try not to get hung up on the apparent contradiction that this road doesn't lead anywhere further south from here) but beneath this facade it hides easily the most unique tourist attraction in town and unusual even by Japanese standards -- yet it mysteriously does not even appear as one of the dozen dots on the local tourist guide map. After the southern side of town got sacked in 1991, 11 of the original houses from the time were preserved here in their half-buried awe. Roughly half are outdoors in a park setting and the other half enclosed in a sort of greenhouse so you can get a good view regardless of the weather. Although at first a pile of rooftops on the ground might just look like the plethora of collapsed houses in rural Japan, the entire house is still there, in one piece but buried, and were buried in an amazingly short period of time by pyroclastic flows from a volcanic eruption. There is information in English, Japanese, Korean. Being a road station, there are restaurants and a fruit stand so it's also a convenient place to pick up lunch.
島原城
An unusually striking castle reconstruction due to its regular, tall yet almost pyramidal, design. Furthermore, it is unique in that the town has clearly chosen functionality over form and forgone the usual garden in favour of placing a carpark on the castle plateau. Urgh. You can park 5 metres from the door. Despite this, the thin terraced gardens behind the castle, overlooking the ocean are packed to the brim with hydrangeas and really pretty.
篠塚邸
Home of a vassal of the Matsudaira. The family owned the house for 11 generations.
山本邸
The family served 13 successive lords of Shimabara Castle, lasting until the end of the Tokugawa shogunate.
鳥田邸
A house for local magistrates.
四明荘
A villa built at the end of the Meiji Period. The garden was built later, including its famous koi pond, which runs under it.
雲仙岳災害記念館
A museum roughly 3 km south of the port. Covers a variety of topics based upon the destruction of southern Shimabara by pyroclastic flows from Mt Unzen's eruption and future disaster prevention.