Himeji, yes. I'm not so sure about Miyajima. For someone fairly familiar with the trains and stations and can make 6 minute connections, Miyajimaguchi is over 2-1/2 hours from Kyoto. More likely, building in more comfortable connections, it is going to be 3+ hours. Then you have to ride the ferry over to the island. Then at the end of the day you do it all over again in reverse. That doesn't leave much time to actually enjoy Miyajima.

If you just want to check another location off your itinerary, it can be done. If you want to spend some time there and soak up the place, better to build in a night someplace closer - Hiroshima or even Okayama. Besides, who would go through Hiroshima without spending at least a little time there?