Not like in Czech Republic, Japan has so many local specialties throughout the nation. Each city or region has the local specialty, what to try when you visited this city or the region.
Definitely you should try kaisendon when you visited Hokkaido, especially in Hakodate. Kaisendon is a rice bowl with various fresh seafood on the rice.
The picture is a kaisendon with a sea urchin, scallop and salmon roe. The restaurants or diners might have kaisendon with fixed types of seafood but most of them offers many types of combinations. Therefore it can be just two types of seafood, sea urchin and salmon roe but you get more portions of sea urchin and salmon roe.
Of course, kaisendon can be eaten out of Hokkaido, especially in northern Honshu or coast of Sea of Japan. However, each city/region has different types of seafood and you should try the best seafood which are caught in the surrounding seas.