Haena State Park in Kauai has a number of attractions, including the notorious Kalalau Trail. It also has a number of attractions that don't require as much effort, including Ke'e Beach, and the "wet" and "dry" caves - Waikanaloa and Maniniholo. Both of these caves are located immediately off of the main road; but only Maniniholo can be explored on foot. Although Waikanaloa is a sea cave it is full of fresh water, and is fed by an underground spring. Despite its pristine appearance, swimming in Waikanaloa is not recommended, as the water is contaminated with leptospirosis, a dangerous bacteria. Finally, both Waikanaloa and Maniniholo each have interesting mythological backgrounds, which you can read about here.
Kauai is known as the “garden island” of Hawaii for a reason: it has a plethora of amazing natural features for its residents, and for visitors. It also has one spot that radiates with an unnatural unintentional man-made beauty: Glass Beach. This beach is a little off the beaten path (but not by much), and will probably never be on any list of Kauai’s top beaches, due to its proximity to various industrial facilities. Nevertheless, if you like to visit strange or unique spots like me, Glass Beach is a spot that is worth a visit. The beach is in close proximity to the industrial area of Port Allen, and during the early twentieth century, was used as a dumping area for various items, including a fair amount of glass.