Things to see from Koyasan - Hiroshima

I'm staying 7/8/2015 at the mountain on Koyasan coming from tokyo-fuji-Kyoto-Koyasan. All my places are reserved so planning the next part of my trip and would drive with me rented car to Hiroshima. I would do a stop as well in Himeji for the castle but the trip is about 6,5 hours so I would like to do a stop in between Koyasan and Hiroshima. any suggestion what place ? Seen I have a car it doens't have to be connected by rail. Hope to receive some reactions. (PS kyoto I do as well before so no need to mention that one either ;-)

I would consider a stopover somewhere on Shikoku. Maybe Takamatsu or the Iya Valley. How much time do you have?

Thanks for your reply, I still have 2 weeks to plan but I would like to do hiroshima, Nagasaki, beppu and then in back-trip to tokyo I would stop again for a few days around Takayama and Matsumoto.

Taking a side trip into Kagawa is a great idea. Visiting Kotohira and going up to see Konpirasan is an adventure in itself, in addition to the beautiful views and interesting shrine:
https://www.youtube.com/watch?v=ley9rVIewoQ
Seeing the Ritsurin Garden in Takamatsu is also one of Japan's best.
https://www.youtube.com/watch?v=afx-Ju52FiY
Okayama also has a number of places, in addition to its castle there is one of Japan's Top 3 gardens, called Korakuen.
https://www.youtube.com/watch?v=5zAI3_N7NEM
The Handayama Garden also in Okayama is an often ignored or bypassed gem that few go to enjoy - it is very beautiful and relaxing.
https://www.youtube.com/watch?v=JGQ8o7k9GbA
Another often missed jewel and off the beaten trail places is Onomichi with its temple tour, and don't miss the best place in the area - the wildly ornate Kosanji Temple.
https://www.youtube.com/watch?v=97Q5EjEOXI8
And just a bit beyond Hiroshima/Miyajima is Iwakuni, which has its famous bridge and nice mountaintop castle:
https://www.youtube.com/watch?v=0COOZXi66mM
There should be enough there to fill up your time.