We just did the tour of Poly cultural. we didn't do any of the extra activities. We had kids with us and they enjoyed it.
I have heard the Hale Koa luau is great.
Of note--it may not matter to you, but the poly cultural center is alcohol free. So if you are envisioning a tropical drink in a coconut with alcohol, you will not get it there.
Totally recommend Duke's

.
We stayed at HHV. It was adequate. But we were on points so had little control over our specific accomodations.
I would be leary of island hopping. In a post-9/11 world, your trip to island hop will take longer. Even if it is just one island over.
If you want HI to be relaxing--I highly recommend sticking to one for your 6 days and packing only when you are (not) ready to return home.
Do not miss pearl harbour and if you care to extend you visit to the base, the USS Missouri is worth a tour as well.
Folks will tell you to hit the market at the stadium for deals. My first trip to Oahu, I Found deals. But on our 2nd trip--It seemed that we didn't find much better than what we could find in Waikiki.
Be prepared for.....RUSH HOUR!!! I did not expect this. So if you plan on renting a car and hitting an excursion at a specific time--be very mindful and plan for rush hour. For some reason--I was not expecting that. But since most of those who wokr in Waikiki do not live there--the highway gets pretty jammed up. Tour groups do plan for that though, so no worries if you are on a tour.
Oahu is not known for waterfalls. They do have one--but they tend (from what I hear) to be inferior to the other islands.
Definitely get an insider guide at the book store and do some research. Then you can find what will really float your boat so to speak.