I too will be going to kauai in June 08' I found a web site online about the Koaie River which is on the Mohihi-Waialae trail, It seems from the reading that it is more than a one day adventure. and it is on the other side of the Island from Poipu, which is where I will be staying. so it is too far away, here is the link
Fly Fishing Trout on the Koaie - Kauai, Hawaii - The Flyfishing Connection
Good luck and let me know if you come across anything else. Also some good brackish water is available around the island.