Aloha! What a wonderful trip you are planning!
I lived on Oahu for just three years, and my son was born there (at the big pink hospital - you will see it!). We lived in Kailua, which is the other side of the island from Waikiki.
We own at Marriott's Ko Olina, which is next to the new Disney resort, Aulani. We go every other year from the East Coast and generally stay two weeks at Ko Olina. For us, a condo with a kitchen and washer/dryer is wonderful, plus having separate bedrooms is great. They have bbq grills right overlooking the ocean, and within a ten minute drive have a Super Target, a Costco, as well as Safeway, so we tend to grill dinners of fresh fish and things often. We do some sightseeing as well, and go by old favorite places like hole in the wall eating places that are nice and bring back good memories!
If you want any specific ideas, feel free to PM me!
I do agree that now, we tend to avoid Waikiki; rather we'll visit it one or two times during a trip. Mostly we enjoy walking around people watching there, but one thing I think you should not miss is to have a cocktail under the banyan tree at the Moana Surfrider hotel. They have amazing music and dancing, and the environment there is spectacular! We used to go stay there for the weekend sometimes, even though we lived 30 minutes away.
For Pearl Harbor, I suggest going early and do not miss that!
We enjoy taking one day and leisurely drive up to Dole, stop and have a Dole Whip, then up north to Haleiwa, stop for lunch, then down the east side and around. The Pali highway is beautiful.
If you're only on Oahu a couple of days, I'd say make it easy and stay in Waikiki. I agree with the PP that the fairly new Embassy Suites sounds good - they get good reviews and are centrally located, and it's new. I've stayed several times at the Hilton Hawaiian Village, and yes, it is huge, the rooms can be fairly nice or kind of crappy (but you're not in the room much anyway) but the big draw is that they have the only real decent pool in Waikiki (most hotels have a little pool since most folks are at the beach or out and about). The beach at that end is the widest, and the water is super clear for swimming and snorkeling.
What we love about Ko Olina (well, everything but here are a few things) is the lagoons are quiet and calm, nice for snorkeling early in the morning, filled with fish. We love the "neighbor island" feel of it, although I'm not thrilled with the Disney resort there now personally - I liked it when it was quieter but that's just me. Since we stay a while, we utilize the condo for cooking a lot (bringing things like spices with us). Also they do have several pools, one with a waterslide and waterfalls, which my kids love, so we hang out at the pool every day for at least a while.
If you are interested, you can find weekly rentals on redweek a whole LOT cheaper than directly through Marriott, or they do have really awesome 4 or 5 night packages I think, like you pay a very small amount, less than $800 I think, if you agree to go to a timeshare presentation. I actually think that's not a bad deal - yes they try to sell you the new point system (but don't buy! We just bought another week every other year resale for less than $5,000 which if you bought the points equivalent to that it would be well over $40,000).
Maui I've only been a few times, twice to the Ritz Kapalua (don't recommend in October, it's cool and windy like crazy windy) so agree Kaanapali or Wailea is better. Also I've never done the drive to Hana, but our friends did and one felt really carsick. I've also never taken the ferry to Lanai - I don't know much about that but it may be fun? I do think I'd love to do a snorkel trip to Molokini, sounds fun.
Sorry to ramble on and on, but we're just planning our trip next June (2013) - already booked our condo! We're just doing 14 nights at Ko Olina like we did last summer (2011) but may do one night on Lanai without the kids (our friends are going to be there as well, so they can keep an eye on the kids for one night). That will be our 20th anniversary (in May)!
Happy planning!
If it were me, honestly, I'd do one island only for that amount of time. You lose a day flying back and forth. But since you already have your plane tickets, I guess that's not really possible. Oahu has a ton to see and do, lots of great things like museums (the Bishop museum is great), the swap meet at the stadium is fun (we go at least 2 or 3 times a visit), etc...