This is a kayak trip down the Wailua River, plus a 1.5 mile hike to the so-called Secret Falls. Some stuff you need to know:
Show up ON TIME and be prepared to spend more time than expected on the water. The tour was supposed to leave at 12:15 and didn't leave until almost 1:00 because so many rude people were late. That meant we didn't finish our hike until nearly 4:30. However, those same people had an event to go to and became very insistent that they needed to leave NOW. As a result, our trip was cut short to get those people back on land. Not because they were whining, but because they'd probably become dangerous and belligerent on the water. So, in other words, don't be a jerk, plan ahead, and your tour will go well.
First thing you'll notice: You meet at a "base camp" near a Hawaiian village, where you get a small safety lecture. The river is right there, but you don't put in the boats there. Instead, they drive all the way out and head to a parking lot/dock off the highway. Duke's HAS to do this. They're not ripping you off. There is some sort of regulation that forces them to put the kayaks in at this site. So don't be surprised if this happens.
The kayaking itself isn't too difficult, though of course, it's two miles and there is a current. The tour guide will let people go ahead to a meeting place and, at the same time, make sure no one falls too far behind. Once at the shore, the hike is of mild to moderate difficulty. The trail gets muddy, you cross some streams, there are rocks to climb over at times and roots to grab on to, etc. In short: wear shoes that are both supportive and water-resistant.
The Secret Falls isn't. There were many, many tour groups there from multiple kayak tours. Generally, people were respectful but it was obviously crowded on the rocky shore. You can swim in the falls, though the water was chilly even in July and it rained on us a bit. It's recommended that you not swim under the physical falls or near the sides because of falling rocks, but you can. No one will stop you.
About Duke's: Evan, our guide, was friendly, down to earth, and patient. He had great forest lore, entertained the kids, and guided us across small obstacles. I'm pretty unsteady on wet rocks and he was very encouraging! The price was good with a coupon: it was ~85 for two people, plus nine dollars for lunch (very simple sandwich, chips, and water). Even if you pay full price, though, you're getting a good 5 hours out of it, which is worth the price.
I had a wonderful time, snapped some great pictures, and got to swim near a waterfall.