Along the southern shore of Kauai is a short, secluded hike along a cliffs overhanging the ocean.
These are sand dunes that have slowly been lithified (turned to stone) over time, and are now slowly being eroded away by the ocean. The bright yellow hue of the rock contrasts amazingly to the deep blue over the very rough nearby ocean that is popular with wind surfers.
The trail starts at Kawailoa Bay, which is gentle enough for some wading, and ends at another bay-type area that looks absolutely prehistoric and barely touched. It's not hard to imagine dinosaurs roaming around the area (I doubt any actually did), but it is truly a sight to behold.
Head South on the Island to the town of Po'ipu, and drive East, away from the Spouting Horn. The road is essentially straight for this part as you drive past the numerous resorts and hotels. Eventually it'll end, and you'll turn on to a dirt road through what is for all intents and purposes a desert. It pays to have 4WD at this point. Drive down this road for approximately 10 minutes and follow the signs fro Kawailoa Bay. You'll soon hit a dead end with ample parking. Inside there there is a hidden road to another area with parking for the Kawailoa bay beach. Park your car and walk east to Kawailoa bay, and you'll see foot path along a ridge overhanging the coast. These are the lithified sand dunes.