Recommended Day trips from Hiroshima

I think  you can do Okunishima and Takehara area in 1 day since their not too far apart. But you'll probably need to choose either Sandankyo gorge or Iwakuni on your last day. Probably don't want to squeeze too much in if your flight leaves that day. Hope my opinion helps.

Between Iwakuni & Sandankyo gorge, which one is the must see place? I can't really decide T_T

I don't know about Sandankyo Gorge but I've been to Iwakuni and the bridge was awesome.

Another idea would be visiting Matsuyama  (Ehime prefecture) by ferry.  It only takes 1 hour 20 min by ferry.

This is the schedule for the ferry for Matsuyama http://setonaikaikisen.co.jp/language/en/superjet/