Hiroshima is an excellent day or weekend trip from Okayama.

How to get there
  • By shinkansen, it takes 35 minutes from Okayama station.
(One way: 6,060yen reserved, 5350yen unreserved. But, in high season (summer/winter holidays) 6,260yen reserved, 5,350yen unreserved)
  • By local train (San'yo line), it takes around 3 hours from Okayama station.
(2,940yen one way)

Atomic Bomb Dome, Museum and the Peace Park
The main reason a lot of tourists visit Hiroshima, and definitely worth the visit. The Peace Park is beautiful, and relaxing. Be warned, the museum is a very sad and sobering experience, but I doubt anyone regrets their visit.
You can get to the museum and park via bus or tram from Hiroshima station. Tram line 2 or 6 (15 minutes, 150 yen). Look for 'gembaku domu mae' - 原爆ドーム前.
The museum is open 8.30-6pm (7pm in August, Dec-Feb 5pm) and costs 50yen.

This island is famous as one of Japan's 3 most beautiful views for 'Itsukushima' shrine, and the iconic 'torii' gate standing in the water. It is home to Mt. Misen, and many wild deer.
It is accessible by ferry from Miyajimaguchi station (Sanyo line, 25 minutes from Hiroshima station, 400 yen one way.) On foot, follow the signs for the ferry port. The ferry takes 10 minutes, and costs 170yen one way. They have JR ferries and Matsudai ferries that cost the same and take the same route.
There are many shops, restaurants and ryokans on Miyajima.
You can go into Itsukushima shrine (6.30-5pm) for 300 yen.
Also, you can climb to the top of Mt. Misen (or, take the two ropeways to the top), it offers beautiful views of the Seto Inland Sea.
If you want detailed information about what to do on Miyajima, check out this guide.

Hiroshima Castle
Hiroshima Castle is a similar visit to any Japanese castle. It has areas for putting on costumes, and has a lot of English information.
It's a 10 minute walk from Kamiyacho tram stop (12 minutes, 150 yen from Hiroshima Station by lines 1, 2 or 6) or a 15 minute walk from the Atomic bomb museum/peace park.
It's open from 9-6pm and costs 360yen.

What to eat?
Hiroshima is famous for 'okonomiyaki', so you should try it while you're visiting (There are two famous styles of Okonomiyaki, Hiroshima and Osaka. Try both and make an educated decision!)

Where to stay?
Hiroshima Hana Hostel - Website
The Hiroshima Hana Hostel is a great, clean and easily accesible place to stay. It is aimed at foreign visitors, so the staff can usual speak good English and it also has washing machines/driers on the roof, and small lockers for valubles downstairs.
You have to make reservations in advance, and you can choose private rooms or dormitories.
You can book online in two ways - making a booking, and doing an email request. I recommend doing the email request if you're booking in advance, then you don't pay anything until you arrive, and you can cancel at any time.