Kauai is the most charming of the four major islands. It's lush and not overly developed. Even though I just visited last year, I wanted to come back and experience Kauai's charm again. Like most charming places (or people for that matter), it doesn't give up all of itself easily. The Napali Coast is stunningly beautiful but not always easy to see. The overlook on the west side if often completely clouded in and is near Mount Waialeale which is one of the wettest spots on earth. From the east, there's no road and one has to both get a reservation and do a difficult hike in.
By the end of my second visit, I felt I'd seen all the things that are not too difficult to see. Kauai is not a large island and the majority of the interior is privately owned. It's a charming and gorgeous island but also a small island.