I lived in Kumamoto before and got to know a lot of great places to visit. But not so many people out there can speak English and the locals are not really aware of the attractiveness. So I might be unique as Japanese who has language skills and local knowledge of Kumamoto.
I love surfing, I traveled all over the world with my surfboard when I was a student. I still travel 2-3 times in a year.
The hidden gem would be the closeness between cities and nature in Kumamoto. You can enjoy huge volcano and hot spring and nightlife in old castle town in 1 day.
A mix of activities between outdoor activity and nightlife in city. It is a fascinating part of Japan!