Spouting Horn, Kauai
Wave action creates pressure inside an old lava tube, causing the water to jet out of a hole on top of a rock bench. In addition to the main blowhole, there are several holes that generate smaller fountaining or at least get flooded from inside as the wave pounds the rock. Catching a good blow in a picture requires good timing and patience, especially with a digital camera.
I'm putting this tip under the Tourist Trap category, mainly because this place seems to get crowded a bit out of proportion with the interest of the attraction itself. There is also a row of souvenir stalls next to the parking lot. I don't know if they offer good deals - one of my guidebooks suggests that this is a good place to buy Niihau shell leis - but they do add to the feel of a "tourist trap".