Concierge.com's insider take:
Kauai's waterfalls vary greatly in their degree of accessibility, but they're all beautiful. Helicopter tours provide the easiest viewing of the grandest inland falls, but you can't feel the spray on your face or swim in the cool mountain pools. (A note about that though: Each year between 50 and 100 people in Hawaii are diagnosed with leptospirosis, caused by bacteria found in mountain freshwater. Don't swim if you have open cuts, and avoid drinking untreated water.) Recent rainfall will affect the appearance of most waterfalls, but the best time to get that postcard-perfect shot is early morning.
Hanakapiai Falls: If you're tough enough to attempt the 11-mile Kalalau Trail, and you have the required camping permit, you can see these dramatic falls—a 100-foot sheer drop down a black lava rock wall. It's located two miles from Hanakapiai Beach, itself two miles from Kee Beach. Reward yourself by swimming in the freshwater pool, but beware of slippery surfaces on the way in and falling rocks directly under the falls.
Opaekaa Falls: Since these 150-foot falls are not accessible by river or trail (after two deaths in 2006, the state prohibited hiking there), you won't feel like a wimp taking your pictures at the crowded lookout. Unlike many waterfalls on the island that are dependent on rainwater, these lacy falls are North Wailua River runoff, so you can count on a rushing flow year-round. The name Opaekaa ("rolling shrimp") refers to the crustaceans tumbling in the turbulent waters at the base of the falls. To reach the lookout, take Highway 56 to Kuamoo Road at mile marker six and drive one and a half miles.
Wailua Falls: These 80-foot falls are easily accessible for viewing and photography (no hiking required). If they look familiar, you're dating yourself—the opening scene of 1970s kitsch-fest Fantasy Island was filmed here. The brave can attempt the extremely slippery tral down to the bottom of the falls and take a dip; hiking behind the falls is not recommended. In ancient times, warriors would leap from the top of the falls to prove their bravery, but unless you are a professional cliff diver and your camera crew and team of paramedics are standing by, don't attempt it. Take Highway 56 north to Maalo Road and drive to the end of the road.