As long as you're going to waikiki......... You definitely have to check out Hanauma bay while your on Oahu. You will practically never get a chance to snorkel in a place like this again. The geography of the sunken volcano has created a place with more sea/reef life than many others. You might get some info off the link below:
http://www.honolulu.gov/parks/facili...ay/welcome.htm
Seriously though, that place was life changing.