LOL I have vague memories of paying a small fortune for a box of Hostess Doughnuts somewhere on Maui!!
My most favorite thing on Oahu is visiting Pearl Harbor. We ALWAYS go there. It is so beautiful and so moving. You must go early, early as the lines are long but it is SOOO worth it. Last time we went in '06 we took DS with us (he was 9) and we also toured the USS Missouri battleship - so it was pretty much an all day thing for us. Well, probably from 7:00 am until 1 or 2:00. The battleship was very cool too. We thought it was a great piece of history for DS to see, although he may be too young to remember it.
We have also done Hanuma Bay - again - GO EARLY!! It is very cool but gets REALLY crowded. You are guaranteed to see a ton of fish.
I went to the Polynesian Culture Center on my first ever trip to Oahu. Personally, I was not that impressed...and have never wanted to go back...but it's a BIG draw for a lot of people. I remember it being a 12 or 14 hour day and we went on some bus tour...perhaps if we were not stuck with this group it would've been better.
Last time we went we booked a catamaran out of Makena (on Maui) that was supposed to go to Molokini but the water was too rough for them to go there. Molokini is usually a good spot if you go early, before the other boats. We went once on a sailboat that was GORGEOUS and it was a fabulous time. She was called the Silent Lady...we always ask around for her & many remember her but they always say she moved to Tahiti and never came back.

She had a great crew. Hummm...I think I may have to go to Tahiti some day!
We have stayed in Ka'anapali and in Wailea...we found when we were in Wailea we were always driving up to Lahaina or Ka'anapali for this or for that so we usually just stay in Ka'anapali now. Last time we stayed right at the Sheraton at Black Rock so we could snorkel off the beach right there. AND it was only a 10 min. walk down the beach to the Whalers Village.
Maria - if you or your husband golf then Maui is the island to golf on too! You can rent clubs to make it easy. I surprised DH with a round for his birthday one year - we went to the Gold Course where the Senior Masters play in Wailea. It was breathtaking. Any concierge desk can help arrange it for you before you go.
AND YES, sunrise at Haleakala is a MUST DO!! We just like to make that afternoon a lazy beach / pool day afterward since you have to get up so early to drive up there. It is slightly less of a drive if you're staying in Wailea.
I would also recommend trying a luau one evening. A luau is basically dinner and a show - they're fun but they're expensive. I would say to steer away from the bargain luau's like the Royal Lahaina (not so good IMO). Try looking for a deal on the Old Lahaina Luau:
http://www.oldlahainaluau.com/flash.html
(Note - it sells out early - reserve it well in advance before you go or you won't get in! It books up like popular ADR's at WDW so call even at like 90 days!)
We like to go to Hawaii every 5 years so we'd be "due" again in 2011 but have decided to wait on Aulani to be ready - and since it won't likely be ready until fall that means waiting for 2012 for us. So instead we've decided to do a Med cruise in 2011 and just delay Hawaii 1 year. We can't wait to go back though!