I agree with Andrea, you don't need a lot of time o Oahu unless there is something specific you want to see. (I live on Oahu) Kauai is lush and green-lots of outdoor activities. The Big Island has a lot to offer also. Waikoloa is the big resort area, Kona is nice, Hilo is close to the Volcano area.
It all depends on what you are looking for.
Rule #1 is don't leave anything you want to keep in a car, if even just a couple of minutes (Oahu in particular.)
PM me if you have any specific questions.