Hiroshima and Miyajima

Hi, I am interested whether it is possible to visit and experience Hiroshima and Miyajima in the same day, or is it worth reserving a day for both?

I am afraid of not having much to do for the entire day in either destination.

If your time is short and you're already in or very close to Hiroshima, yes, it's possible to do both in a day.

Start at the Peace Park then take the AquaNet waterbus from there to Miyajima and spend the rest of the day there.

If you want to see Hiroshima Castle & Shukkei-en Gardens as well then time may run out for you...

It depends on what you want to see and do. If you only want to see Peace Memorial Park in Hiroshima and Itsukujima Shrine in Miyajima then you can do it easily in one day.
If you have a JR Pass then you can take local train (JR Sanyo line) from Hiroshima station to Miyajimaguchi station, and then hop on a JR ferry to Miyajima. It will take a little bit longer then the direct boat from Hiroshima, but it will save you some money as both train ride and ferry are covered by JR Pass.

Indeed. It depends where you are coming from. If from Osaka or Kobe it is possible if you take the morning train. From Tokyo it is not possible.

It is possible to see garden castle museum miyajima in a day. But it depends on your travelling pace. If walking the length and breath of each attraction and a few photos here and there is what you consider travelling it is doable. If reading every sign and taking photographs with different lighting, exposure and lighting is your thing.... you need more time.