Spent a week on Maui for my honeymoon. I second the sunrise at Haleakala.
Since you're on a cruise, you won't have time to do the complete Road to Hana, but we loved it. Driving through different terrain (rainforest, hills, narrow bridges, etc.) was great.
Snorkeling/whale watching/golfing, etc. all great. Some REALLY good restaurants as well - David Paul's in Lahaina was our favorite!