This 5 hour trip starts with a Kayak trip up the Wailua river, through tropical jungle. Several miles up the river you paddle in to shore, secure your Kayak then hike inland through a tropical rainforest to Secret Falls, a 100 foot waterfall that you can swim beneath. Along the trail you will pass other smaller, but even more beautiful falls at hidden places. While you can rent a Kayak and do this trip without a guide, it is probably easier to make this part of a guided hike/trip - that way you don't have to worry about the Kayak when you leave it tied up during the hike.
Our guide was incredibly friendly and knowledgeable, which made it all the more entertaining. He told us about all the different fauna along the way. Of course I forget most of what he told us, and for all I know he could have lied about most of it anyway. Nonetheless, it was interesting while we were there. The hike itself was along a well-worn path through fairly thick rainforest. At the end of the mile-long hiking trail, the path opens up to the waterfall. We brought lunch, ate, and then went swimming underneath the waterfall. Yes, I was worried about leptospirosis, but I quickly got over that when I jumped in and my testicles shriveled up to the size of raisins. The water was unbelievably FREEZING. It was still fun, though. Definitely one of my favorite hiking trips I've done here.