Get reservations in advance for the black sand beach past Hana. That beach and Hamoa Beach, which is also in the general area, are my two favorite beaches on the island.

I'd also buy the Maui Revealed book.

I took sunrise bike tour down Haleakala on my first trip there and that's my favorite thing I've ever done. It involved leaving the hotel at about 2am and it was very cold waiting for the sun to rise, but it was worth it.