The Waimea Canyon is one of Kauai's landmarks. You can drive to several viewpoints or hike along one of the canyon's many trails.
Turtles and seals like to lay on Poipu Beach around dusk, and the beach is a popular surfing spot for locals. Grab a hotdog from Puka Dog (the famous Kauai hotdog shop) and watch the turtles come to shore, or grab dinner from the Beach House Restaurant and watch the surfers catch some waves.