Perhaps the starkest reminder of the destruction visited upon Hiroshima in WWII is the Atomic Bomb Dome. Built by a Czech architect in 1915, it was the Industrial Promotion Hall until the bomb exploded almost directly above it. Everyone inside was killed, but the building was one of very few left standing near the epicentre. A decision was taken after the war to preserve the shell as a memorial.
The building has since become a haunting symbol of the city, and was declared a Unesco World Heritage Site in 1996. Try to wander past in the evening when it's quiet and the propped-up ruins are floodlit.