We have taken this ATV tour and had a blast! The guides were knowledgable and it was a great way to see part of the island that few get to see. Recommended for the adventurous and wild!
"Going to Kauai without taking a helicopter flight is like going to see the Sistine Chapel and not looking up."
If you only have a day to do this and you want a difficult hike, only hike to hanikapiai falls (4 miles each way). The full hike is 22 miles round trip and requires camping (permits required).
This is also a good resource for beaches!