We go to Hawaii every year but it's been 10 years since we've been to Oahu. However, we always enjoyed the night show at the Polynesian Cultural Center. Other than that, I too would recommend jumping to another island. Maui and Kauai are our favorite islands. Road to Hana is pretty awesome, if you do it, pay a few bucks and download the app Road to Hana Maui GyPSy Guide. It's a guided tour of the entire road that's GPS driven so it will tell you exactly when to stop to see waterfalls, gives you TONS of history, points out different things, etc and if you want to stop at a spot for some extra time, it picks right back up where you left off because it's GPS based. They have a version for the entire island too if you're wanting to see other things. I believe they have versions for the other islands as well. Worth the purchase.
Really, when we go, we don't plan anything. We may have a few things we'd like to hit at some point, but we go to enjoy it. Our funnest trip was one that we would go drive around Kauai and find random beaches and just go swim. Always had the entire place to ourselves, super relaxed, etc.
The Big Island tends to be the cheapest. Still a good time but the Kona side is ugly and dry (black lava rocks every where, very little vegetation, etc). Being dry has the advantage of just that...you're not dealing with rain. It's also something to keep in mind that just because it's raining, doesn't always mean no beach, pool, or whatever. It's still warm and every bit as fun as long as it's not lightening.
As mentioned, multi-day trips around any of the islands is way too long. Even on the big island, I could see one day making your way over to the other side and a day coming back, most the other islands, depending on how much you stop, it's a half day to a day tops.
We've been debating when to do our trip this year. We usually go some time in April or May but this year I think we're going to go a little later. If this is your first trip over, it doesn't matter what you do, you'll have a good time and you'll have a million things you didn't do that you wish you could have. Most important is to remember that everything/one is on island time and you just need to kick back, relax, and enjoy it. Oh, and don't be surprised to see 40mph speed limits on the freeway