Miyajima Island houses the Itsukushima Shrine, a UNESCO World Heritage Site and is very accessible from Hiroshima. I use the JR Seishun 18 Pass* to get there, and it allows me to take the JR ferry across as well!
*Seishun 18 Pass provides 5 days of unlimited JR rides. They do not need to be consecutive days, but is available on select periods only. It targets high school students to use it during their holidays but is available to everyone.
I had a late start going to Miyajima and so I conclude that a half day is sufficient for exploring the island. If you plan to go to the aquarium and hike up the mountain as well, you’ll want more time.