Yes, this whole trip was done in one day. I arrived in Hiroshima at around 9am and left at around 6. Pretty much everything closes on Miyajima island at around 5 except for a hotel or two there. I think the last ferries are around then, too.
We left Hiroshima after visiting the museum and seeing the statue of Sadako Sasaki who was 2 years old when the atomic bomb dropped in Hiroshima. When she got a bit older, she was diagnosed with leukemia due to the radiation. So, so many children from Hiroshima and Nagasaki also had leukemia. There’s a Japanese story that says that if you can make 1,000 paper cranes, you will get a wish. Sadako tried her best to fold 1000 before she died but only got to 600 and something. She was 12.
We departed Hiroshima for Miyajima at around 12pm and got there a little after 1 pm and the tide was finally low enough to walk out and touch it at about 5:30 pm. So about 4.5 hours!! We spent that time walking around the island, looking for sea glass, shopping for souvenirs, and relaxing in cafes.
From the Kobe area, it was about 2 hours but I left too early for anything but local trains to take me too Himeji (where I changed to a shinkansen). So, it could be an hour and a half. My friends who came from Osaka said it was about an hour and a half, but from Shin-Osaka, it’s a shinkansen the whole way! 🙂 It makes a great day trip! You can also go by night bus, I think!