Sounds like a fun trip. To get from Onomichi to Izumo, I'd recommend continuing to follow the coast around the toe of Honshu to Hagi. Stop also at Iwami. Then onto Izumo. Most of the time, you'll find these places to be very uncrowded. Definitely off the beaten path for non-Japanese tourists. You may find that you're at the mercy of infrequent buses to get from the stations to some of the sites.

Some tips on the region in this thread:
http://www.flyertalk.com/forum/japan...y-6-japan.html