I was just in Kauai a couple of weeks ago, though sadly for only 3 days of a weeklong trip to HI.
My only recommendations are:
-when going to Waimea canyon, go past the "Waimea Canyon Lookout" to mile 18. There is the Kalaulu (sp?) Lookout. Park there, then walk about 1.5 miles down the road marked "closed." It's worth it.
-check out the harbor and bay at Hanalei. If you can get there for a sunset, it's beautiful.
-drive all the way around past Princeville and Hanalei, too - there are a couple of cool beaches there, and the silhouettes against the sky can be amazing.
-Bring your polarizer and grad NDs if you can.
sigh. good times.