Wailua Falls, just north of Lihue, is easily accessible. You can view these falls from the road, with no hiking necessary. You may recognize them from the opening credits of the long-running TV series "Fantasy Island" as you see them cascade 80 feet in two separate streams.
Not far from Nawiliwili Harbor, you'll find Alekoko Fishpond, a nearly 1,000-year-old example of ancient Hawaiian aquaculture that's on the National Register of Historic Places. Legend says the Menehune people built the pond in one night by lining up and passing stones hand-to-hand for 25 miles.