Congrats on going to Kauai!
My wife and I spent 8 days at Princeville in April of 2008 and 2 days at the W in Honolulu for our 2nd Honeymoon. I wish we had just gone straight to Kauai!
I can't say enough good things about Hanalei and the Princeville. Although it wasn't a St. Regis at the time. We were upgraded nicely, and used the 4 day, free night promo and 50% Plat Amenity at the same time. It is a gorgeous area and even though the town of Hanalei is small, there are good grocery stores and plenty to see.
We found the food at the hotel somewhat expensive, so having great grocery stores (wine, cheese, fruit) and great breakfast spots 4 minutes away was nice (Hanalei Cafe ^) We spent 4 hours hiking along the Na Pali Coast, and it is an invigorating climb/walk (mostly up). It started to rain after we were out for 2+ hours, making some of the walk back VERY slippery on wet roots and clay like soil. So watch the weather.
We did spend about 2 days on the South side of the Island at beaches and exploring. It gets pretty remote and a lot of red clay dry plains on the west side. A completely different look to the Island. I can't say I found any of it to be very scenic, but we thought it was a lot of fun. Waimea is really quite a boring town. We did go to a coffee grower and take a tour which was fun.
I do highly recommend talking the Blue Hawaiian Helicopter Tour early. It seems expensive, but there is no better way to se Na Pali (and the whole Island) than in the air and I found Blue Hawaiian to have the best fleet of Helicopters. If you go to their website, you can see some fantastic HD video of parts of the ride (esp Na Pali Coast Cathedrals) I don't regret any of the money we spent doing it and the pictures and experience on the day left us feeling fantastic! For how long it took us to get there from Austin, TX, seeing the glory of all of that Island from above is worth 2 beaches for sure!
I love the Northern Beaches. They are surely some of the nicest on the island. The snorkling was fantastic. I really liked the beach at the Princeville. You can rent masks and snorkle in town pretty cheaply too.
Congrats on your honeymoon and welcome to FlyerTalk!