Check out their version of the Grand Canyon, quite impressive! Also if you are not much of a hiker, I would recommend a helicopter tour---you get to see alot of stuff in a couple of hours.
Are you staying on the South side of the island or in Princeville? I stayed in Princeville which is more like a rainforest type environment, but after visiting the south side which is sunnier, I wished we would have stayed there instead since there are also seem to be more things to do down there. Also watch out for the wild chickens which cross the road and the people who are strung out.