You might be thinking about what to do during Golden Week and you don't have a car. Here is a suggestion.
Take the bus from Kotsu Center to Kumamoto Shinko, and from there the ferry to Shimabara, Nagasaki. The ferry ride in itself is fun - bring some bread to feed the seagulls. It's about a 40 minute cruise across scenic waters - pay attention to Mt Fugen, which erupted within my memory, killing 43 people.
The Shimabara ferry terminal is within walking distance of the town. It was a samurai town, so there are many bukeyashiki (samurai residences) with interesting architecture which you can tour, lined by small canals they used to tap for water. These canals are now filled with koi, so save some of the seagull bread to feed them.
There is a small castle there, but it is not worth visiting.
The town is the center of the Shimabara Rebellion (1637), an uprising by the local Christian population, encompassing Amakusa, against the Tokugawa Shogunate. Guess which side won. https://en.wikipedia.org/wiki/Shimabara_Rebellion But the Shimabara folk have never forgotten this and are fiercely independent. - William