Aaron, I have to agree with Joseph's post regarding scuba diving on your trip. It slipped past me when I read your post. You are better off snorkeling, than scuba, IMO on Kauai. A few reasons come to mind, the main ones are; the surf is rough at high tides, the boat dives are $$, you'll have more time for other stuff as scuba takes up pretty much a whole day for a couple of hours, at most, in the water.
Spend a few days on the NorthEast side of Kauai, in Kapa'a, Kileua, Hanalei, staying over in Princeville(cheapest)taking day trips from here to the afore-mentioned towns for enjoyable site-seeing.
THEN, head over to the West side of the Island to Waimea Canyon, and camp out and backpack for the last few days; ending with a day in PoiPu hotel for a relaxing day and great little market by the Sea for souviner hunting.
If you are flying through Honolulu then let me know, otherwise Lihue has some nice resturants on the water, and a few things worth seeing, but is the Main city on Kauai.