Definitely something you need to do while in Maui. There is absolutely nothing like it and it is so different than the rest of the island. Very, very beautiful!
I highly recommend getting a copy of Maui Revealed (otherwise known as the blue book) as they do a great job reviewing the waterfalls and giving great directions on where to stop and what landmarks to look for in doing so. You will have a hard time if you tend to get car sick but you are driving slow enough that it may not matter (like I experienced).
A few specific recommendations:
Just before mile market 29 is the Nahiku Marketplace. We started early from Wailea (around 7:30) so not many places were open for lunch. Fortunately Island Chef was just starting to cook and we had the best fish and chips we've ever had. Super generous portions - big enough to share between two people. And included was fresh passion fruit on the side.
We also ended up driving all the way around, which is not recommended by some of the guidebooks and the car rental companies. We asked some locals and the tour guides who had large vans and they said it's not a problem. And they were right (which my husband was really happy about)! Just take it easy and slow down in the areas it isn't paved.
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