We decided kind of last minute to go on a kayak trip to Secret Falls (I guess they're not so secret anymore since a few dozen companies organize kayak trips to the falls...) and fortunately they still had some spots available. Book one or two days in advance, and they'll schedule you in.
For the money, you'll get a guide, some water, snacks and a sandwich. The sandwiches are good by the way!
They use double kayaks, which are ocean kayaks. The chance of tipping these over, is rather small. But it can always happen, of course. Wear a life vest if you're not a comfortable swimmer.
After a lesson in paddling techniques, you depart on a 2 mile kayak trip on Wailua river. Going upstream is quite easy, as you are most likely to have the wind in the back. About 45 minutes in, the kayaks are secured on land and you will hike to the falls. The hike is an easy one in dry periods, but could get muddy after rainfall. Wear shoes with good grip and that can get wet, as you will have to cross a knee deep river twice.
The falls are quite nice, although when we got there, little water was flowing. You are advised not to swim in the pool, but let's be honest: you just have to do that. Watch out for things dropping of the falls and just let the waterfall rain down on you.
The way back can be a bit harder, as you'll be paddling into the wind.
The guides know their way around the forest and can tell you very interesting facts about all the fauna and flora. We had Zach as a guide, and he did an excellent job. He never rushed us, but still got a nice pace going, returning us back to Wailua within a very reasonable time and without being exhausted.
If you have half a day of spare time on Kaua'i, this would be a great way to spend it.