I enjoyed the ferry ride from Hiroshima to Matsuyama, but it does make more sense for you to retrace your steps from Matsuyama to Takamatsu and cross the Seto Ohashi back to Okayama, as long as you're not going to southern Shikoku.

Another interesting place to see near Okayama is Kurashiki, which has a well-preserved Edo-period quarter along a canal.