If you are staying Osaka or Kyoto, it is possible to make a day trip to Hiroshima. In Hiroshima you can visit two World Heritage sites.
Hiroshima Peace Memorial (Genbaku Dome)
Hiroshima Peace Memorial, also known as the Genbaku Dome, was one of the few structures to survive the atomic bombing of Hiroshima on August 6, 1945. The dome was originally the Hiroshima Prefectural Industrial Promotion Hall and has been preserved in its ruined state as a reminder of the destructive power of nuclear weapons. The building was designated a World Heritage Site in 1996.
Itsukushima Shrine is located on the island of Miyajima in Hiroshima Bay, this shrine is known for its iconic “floating” torii gate that appears to be standing in the water at high tide. The shrine was first built in the 6th century and has been rebuilt and expanded several times throughout history. It is considered one of the most scenic spots in Japan and was designated a World Heritage Site in 1996.