Just a quick note recommending the Hiroshima Peace Memorial Museum. Terribly moving and at only 50¥ entrance fee possibly the most ridiculously underpriced educational experience I've ever had. The Sadako exhibit was quite touching and there was a man visibly crying by the end.
It's not a large museum but, as with most Japanese museums I've visited, it gets the point across efficiently and effectively. About as visceral as the Holocaust museum, though narrower in scope.
Towards the end there is an area set up to sign petitions for peace, I can only imagine the shrunken hearts of anyone who doesn't sign after seeing the burnt children's clothes and twisted tricycle artifacts.