Oahu. Yeah, it was tough, I can tell you.
I thought my son wouldn't enjoy himself (after all, he's been to wdw 4 times, and dl once for vacation) but I got him a boogie board and he was just fine. He seemed to enjoy just catching the waves as the crashed on the beach, and rolling around in the sand. (it is hard to get the sand off of him).
We did take him to hawaiian waters adventure park, on the last full day we were there. It's pretty small scale, but there are no lines for any of the slides.
I'd been planning this trip for the past year, and no amount of internet info or pictures prepared me for the beauty of hawaii. We were able to go to the north shore and spent a couple of days there (the one beach we went to had no one on it-just some bitty who didn't like the "irritating tourists with the annoying child"-the water was so clear-I had to borrow my son's snorkel set and went looking for fish among the corral). I did get out and walked around shark's cove, and stopped and looked at sunset beach (the waves were about 1-3 ft-nothing like wintertime).