Overall, this was an incredible trip. It was so much different than other Hawaiian islands, like Maui; it was much more calm and peaceful. The food was excellent everywhere we went, and the people were super accommodating.
If you are semiathletic, I would suggest doing at least part of the Na'Pali Coast hike. To the first beach, it is 2 miles one way. The views are incredible and the hike was fantastic. Grab a walking stick from the pile of wood before you begin--it really helped us out. Pack water and maybe even a snack for the beach at the end. You will definitely want to walk around or just sit on the rocks and relax-- it is a tough hike. We were told that there is a waterfall 2 miles further, but that the hike was extremely difficult. Needless to say, we didn't venture further than the 2 miles. While hiking it got so hot that we could actually see steam coming up from the dirt. We are not golfers, but have heard (and saw) that the Princeville offers quite a course (and an expensive one at that).
The helicopter ride was incredible, and so worth the money. The pictures we got were incredible. It affords you the opportunity of getting amazing pictures and gives you the chance to see areas of the island that are not reachable otherwise. We found that buying a higher-speed film alleviated any bumps we had while in flight. I think that 90% of the pictures we took that day look like postcards. It is expensive though, but if I had to pick one paid-excursion while there, this would be it. This is definitely an island where you can get in the car and get lost. The mountains are in large supply and are all lush and beautiful. Make sure to stop for shave ice at the Wishing Well truck--it's famous! This island is about relaxing and being in nature. If you are looking for tall buildings, look somewhere else.
You must rent a car if you plan to leave the resort. Everything here is very spread out, and taxis are sparse. Walking really isn't an option either, considering you are basically surrounded by mountains. As with the rest of Hawaii, the car companies make you pay a fee per driver--so if you don't plan to drive, don't put your name on the rental contract. One downfall to staying on the north side of the island is the drive to the airport--it's long and the flights back to the mainland (like N.Y.) leave late at night, so you are driving on unfamiliar territory with a time line. Keep in mind that since Kauai is the most lush--it rains a lot here. Plan for it; and if you have specific things you want to do while there, take advantage of the nice-weather days. Especially since you never know what the next day will bring.