I really enjoyed the Kalalau Trail. It is a nice, but long and at times challenging, hike to a truly spectacular beach with a two-tiered waterfall. It is on Kauai. It is the only place I've encountered a hammerhead shark, which was cool. Camp there and make the most of your vacation dollar in Hawaii. The lower waterfall is used for showers, and they do have composting lavs, so although primitive, it is not stinky, uncomfortable, rustic camping.
I will add that I would rank the beach as my second favorite beach in the world after Easter Island's with the Maoi.