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.
Directions: The cave is located at Mile Marker 10, immediately off of the 560.