I hiked the Na Pali coast this summer, and it was great. As far as the big island goes, Mauna kea is a 10 minute hike, and it is amazing at sunrise, a must do if are into mountains and on the big island. You'll probably be fighting crowds at sunset. We had it to ourselves at sunrise. The road up is great, 5 miles of gravel, the rest is paved, and the gravel is good. You could also hike Mauna loa, it is about 6-7 miles and around 3,000 feet. Probably many others, that is what I know of. It doesn't get much better than wearing your down jacket in Hawaii in August, and swimming at a beautiful beach 2 hours later.