Kaua'i is astonishing. From a geological standpoint, it's the oldest of Hawaii's main islands, created from it's volcanic history. The rugged coastline placed against the effervescent sunsets shows off the beauty of the "Garden Isle". Islanders here respect their nature: no building is allowed to be taller than a coconut tree, it's impossible to circumvent the island, and there is no ferry system to the island (the residents blocked the proposal because of environmental concerns). You can dive into shattering blue caves, wind surf while listening to the vibrato of humpback whales, and hike through Waimea Canyon, adored by Mark Twain and all Jurassic Park fans.
(Note: sorry for the bad photo quality, all of these were taken with a very old iphone back in 2011!)
All of Hawaii is stunning and you can't go wrong with any of the islands you choose, but Kaua'i will welcome you with the friendliest of people and a beautiful appreciation for life.