More to do in Oahu, particularly in the evening on Kalakaua in Waikiki. You can circle the island in half a day, but be sure to check out Hanauma Bay. One of my favorite beach areas is just past Hawaii Kai and Hanauma Bay -- it's called Cockroach Cove (it's a little cove where you can jump off short cliffs and swim with the tortoises). There's also Sandy Beach, which is nearby. On the other side of the island is the North Shore, where the waves should get mammoth-like starting in November through February. Enjoy!
BTW, interisland flights are cheaper than they used to be ... try Go airlines for those . ....
http://www.iflygo.com/