For about five hundred years, roughly from the fourth to the ninth century, people sailed out into the Genkai Sea to leave offerings to the three goddesses of Munakata, and what they left is why this World Heritage site exists. The sacred island of Okinoshima is only part of it. The listing also includes the Munakata Taisha shrine complex on the mainland and on the island of Oshima, and a field of burial mounds called Shinbaru-Nuyama, where the Munakata clan, the local sea lords who ran these rituals, were buried.
The mainland part is open and easy to enjoy. On the coast of northern Kyushu, in the countryside between Fukuoka and Kitakyushu, you can ride the minor roads along the shoreline, with views of low islands, fishing harbours and calm bays. The shrine called Hetsu-miya has a broad wooded precinct and a hall for worshipping the goddesses of the sea, and there is a museum, the Shinpokan, which keeps the treasures excavated on Okinoshima: gold rings, glass cups, mirrors from China and the Korean kingdoms, a whole shipping history of the East China Sea displayed in cases. It is well worth an hour, especially if you want to understand what the island really was.
The burial mounds, a short ride away, are low and grass-covered, the sort of place where a plain-clothed dog walker is more likely to be met than a tourist. Nothing here is theatrical, which is exactly why it works: you are in a landscape lived in by one clan for centuries.
For logistics, the nearer towns are well stocked with fuel and food, and the Fukuoka region is famous for its eating. Order a bowl of tonkotsu ramen in Hakata, or look for mentaiko, the spicy cod roe, and grilled seafood on the coast. If you are curious about the sea itself, ferries cross from the small port of Konominato to Oshima in a few minutes.
The best season is spring or autumn. In typhoon months the Genkai Sea is rough and ferries are cancelled, while winter winds come hard from the northwest and make the coast road a cold one.