The absolute best things to do in Kauai are to take a helicopter ride (the longer the better), hike through the one, or two, of the many trails, and snorkel of course. If budget allows definitely go on a short boat trip of the southwestern shores which cannot be seen up close by any other means. The waterfalls are beautiful to see as well; I recommend up close by hiking or taking a small boat through the rivers and canals.
Quick Tips:
Eat the banana pancakes and local Asian cuisine; yummy! Take up the sun and enjoy the local pizza sites; some of the best pizza I have ever tasted was in Kauai (and I love pizza)!Best Way To Get Around:
Rent a car and then hike it.