In Hiroshima, Peace Park and the Peace Memorial Museum are a must see. They're probably some of the most moving experiences of my life as far as travel goes- right up there with the Arizona Memorial.http://www.pcf.city.hiroshima.jp/top_e.html
If time permits, it is a short train & ferry ride to Miyajima, which has some incredible shrines and temples.
The O-torii(gate) of Miyajima is a famous landmark.
There are loads of hiking trails on the island which are fun to explore. Watch out for the monkeys though!
I was fortunate to be stationed at MCAS Iwakuni for a year. Japan is a beautiful country with fabulous people- enjoy it!