There is a day pass available for the trams + non-JR ferry to Miyajima:
http://www.hiroden.co.jp/en/ecotickets.html
A bit slower on the tram vs. JR train to Miyajimaguchi, but cheaper as well.
If time is at a premium, you can also use the pay-as-you-go ICOCA card recommended above for Kyoto on public transit in Hiroshima as well to avoid having to spend time buying a ticket.