Kayak Wailua, Kauai
Kayaking up the Wailua River, the same river the Fern Grotto boats drive and water skiiers fly, to a 100-foot waterfall is apparently a must-do for many tourists. By must-do, I mean while you are kayaking, you see the other kayakers all over the river, on the trail, and at the waterfall. It's not that expensive (compare to all the other tours on island) at $40-$50. While it is really cool, I hate things that are crowded with tourists, and this one usually is.
Unique Suggestions: Do not try to go on a super sun-shiney day. It was by chance that when I went, we put off kayaking to the very last day of our trip. Unfortunately, it had poured the night before and was raining in the morning, and no one would rent us a kayak. So, we started looking at the tours. No one was going due to a flash flood warning that had popped up that morning. Except one company, Kayak Wailua. By the time the tour was to start, it was dry and sunny, but everyone else had cancelled. So, we were the only people on the river, on the trail and at the waterfall. And the waterfall, which sometimes barely reaches the pool at its base, was thundering to the point you couldn't look up at it as you got so much spray in your eyes. If you go, I whole-heartedly suggest trying to find a group going the morning after a big rain. If we had gone ourselves a different day, there still wouldn't have been as few people as there were with our tour (14 tourists, 3 guides). It was one of our best experiences, purely by chance.
I'll get the pictures up later, but here's the website of the company that went when no one else was going...