Itsukushima Shrine

In 1168 Taira no Kiyomori chose Miyajima as the site of his clan's family shrine. The halls and the famous torii gate stand over the water, so at high tide the whole shrine seems to float in the sea. Check the tide times before you go, because the effect depends on them. It became a World Heritage Site in 1996, and it is the place to see the late Heian period, when the Taira were at the height of their power, before samurai government moved to Kamakura. The shrine is a ten-minute walk from the Miyajima ferry pier, so it pairs easily with a day in Hiroshima.