Opaekaa Falls is one of Kauai's easiest waterfall stops, viewed from a roadside lookout above the Wailua River valley. The falls drop through green forest, and the opposite side of the road gives a separate view over the river bends and valley.
It is not a wilderness hike, but that accessibility is useful on an island where weather and terrain can complicate plans. Visit early for lighter crowds, and resist any temptation to scramble beyond signed viewpoints.