Drive west on highway 30
One of the best things to do in Maui is just to drive west on the highway 30 and stop in different places. I have to warn you. There is no really signs on the road for these places, but you will see around the highway some cars stopped at the side of the road, and my advice is to stop there to look around and you will discover great beautiful places.
Going west, drive until the highway it is not a highway anymore but becomes a road. It is around mile 43. Turn around and drive back stop when you see cars on the side of the road. The first think you can find, around mile 42 ½ is the Olivine pools. Something that most books will not tell you is that most attractions are not just some steps from where you park. Most of them require you to climb down and up on rocks. So please take some tennis shoes or water shoes. Flip flops will be slippery. It is quite an exercise, so take water with you too.
The drive with stops will take around 4 to 5 hours, which will give you time to schedule a nice dinner at Lahaina.