Well, you probably already know about Lanikai and Waikiki beach.
Lanikai is the most awesome beach on Oahu. It's very secluded, has very fine sand, and the waters still enough for anyone to swim in. Kailua beach is awesome too and has kayaking and wind-surfing.
Diamond Head, Honolulu is way kewl too! It's 350 acres with hiking trails and you can see the crater, landmarks and tons of awesome stuff.
Hanauma Bay nature preserve, Honolulu is really kewl for snorkling. The fish are awesome and so is the wildlife.
Oh and you should check out the La'ie, Maunawili, Manoa, and Waimea falls. They have trails for all of those.
I don't know that much about the other islands so somebody else needs to kick in about those.
Have fun and I hope you will post pictures!