You must see Old Lahaina and Molokini. If you have time, take the "Road to Hana." I rate it a so-so, but others in the party enjoyed it. I thought it was beautiful but long (about 6 to 8 hours) and boring. We flew over to Oahu for a day, rented a car, did the tour at Pearl Harbor (long, but worth it - go as early as possible), went to Dole Plantation (a little cheesy), and drove up to the North Shore.
Quick Tips:
If you go to Pearl Harbor, be there when it opens, as the line gets longer as the day goes on. If you are able, try the surfing lessons. My son had one-on-one instruction from a former pro-surfer for 2.5 hours. He was up on the first wave. (It was only .)Best Way To Get Around:
Rent a car so you can go where you want to, when you want to. I would not use any other option. The roads are not crowded and you can stop anywhere. The night I arrived, I saw a police car at an accident (I found out the person died - local DUI) but hardly ever saw another.