Drive up Haleakala in the am before the clouds set in, dress warm! Maui Ocean Center is great and if your a diver you can do the shark dive, no cage witgh 30ish sharks. Ive done it twice and it is fantastic! If your not a diver do the intro dive or maybe even get certified. You can do the class work at home and do the dives there. Screw Oahu, it sucks a$$. The road to Hana is a nice drive, go all the way around and ignore the rental car companies warnings, they are out dated, the road is fine. Find a couple of hours to wander Makawao, cool old ranch town (thats where I lived). Avoid the plantation train, it's a joke. Lahaina deserves most of a day wandering also.
Have fun!
Aloha