Have been there twice...you'll have a great time. Agree with the other posted about Oahu..looks like you have your time split up well. If you can find these at a local bookstore I'd recommend picking up these for Maui & Kauai. They are fantastic...great insight on things to do, restaurants, etc.
http://www.wizardpub.com/Kauai/kauai.html
On Kauai the canyon was amazing...definitely have to make sure you see that.