Can I go with you?
My wife and I have been there twice. She doesn't kayak so my options were a bit limited. We did get on the Wailua. Its a flat water paddle. We rented a 2-man sit-on-top from Kayak Kauai. These guys were cool and laid back.
I recommend the Secret Falls paddle. Once you get to the beginning of the hike its a blast. The trail is something out of the Three-Rings movies and we were like kids stomping in the mud. The stream next to the walk is interesting, I saw a couple of holes where I would have liked to have had my boat. http://www.kayakkauai.com/secret.html
I'm not sure if they have any real white water.
We also found the irrigation ditches up in the hills. Ran into a couple of locals who gave us boogie boards to float through the tunnels into some cool gottos. Be careful exploring these, some of them end in waterfalls or drops and you don't know it until you are right up on them. Just stand up. This was the coolest part of the trip. We're going back in May and will be exploring some more.
Also look for the Powerline hike. It is over near Hanalei. Its a long one, but you go through two tunnels and come out in the Hanalei Valley up high. Rivers look interesting (Hair when its raining) here, but I wouldn't want to hike a boat up there.
PM me if you want other info. I love Kauai