A helicopter trip (we did Jack Harter) and morning sailing trip with snorkeling from the north shore are especially recommended, if possible. I agree that the Kauai Revealed book/kindle was a valuable resource for information.
Here is a great 4k view from a “doors off” helicopter trip in Kauai – 10 minutes or so in, it gets to some nice waterfalls - 14 minutes or so in, it gets to the Waimea Canyon – 23:30 they get to the coast and the Na Pali coast has some pretty unique scenery:
https://www.youtube.com/watch?v=JL3NA2lyDYM
I haven’t watched as much of this one, but it gives you an idea of a boat tour from the North side along the coast:
https://www.youtube.com/watch?v=Rx1OEVWrjTI
The scenery is “best” with the boat tours from the North shore, which are typically morning tours as the water on the North side can get choppy in the afternoons. The afternoon/sunset boat tours sometimes have better lighting for the Na Pali coast but if you only do one boat tour, again, I’d vote for the North shore. Captain Andy's afternoon tours are extremely popular, so book well in advance (which has a little risk due to the variable weather).
You can also google images of Waimea Canyon (spend a day driving - you can see a lot from the overlooks and short walks) and Na Pali coast to see more. Hiking is a great option in several places on the island.
https://www.gohawaii.com/islands/kauai/things-to-do/land-activities/Hiking
I thought the best snorkeling was in Poipu but the rocks on the coast are sharp so be wary.
It’s an amazing place to visit…
