Let's put this in perspective: even if you had only one day on Kauai, we'd still recommend heading to Napali Coast on Kauai's northwest side. Once you're there, you'll soon realize why no road traverses this series of folding-fan cliffs. That leaves three ways to experience the coastline—by air, by water, or on foot. We recommend all three, in this order: air, water, foot. Each one gets progressively more sensory.
Napali Coast runs 15 miles from Kee Beach, one of Kauai's more popular snorkeling spots, on the island's North Shore to Polihale State Park, the longest stretch of beach in the state, on the West Side of the island.
If you can't squeeze in all three methods—air, water, and foot—or you can't afford all three, we recommend the helicopter tour for those strapped for time and the hiking for those with a low budget. The boat tours are great for family fun.
Whatever way you choose to visit Napali, you might want to keep this awe-inspiring fact in mind: at one time, thousands of Hawaiians lived self-sufficiently in these valleys.