Just got back. Best place in islands in my opinion. Very lush, less touristy that other islands. Very mellow. Biggest probem is lack of hoppin' night life. You have to go to the big hotels for that. But if that's not a must, you will love it. Doesn't take long to get from one end of island to the other. I stayed at Poipu Beach which is nice. Lots of good board and body surfing nearby./ Lots of sea turtles. But North Shore also has some great places.
The two best sides I had were tubing and tour of Na Pali coast with Kauai Sea Tours, though there are many others. You'll see lots of dolphins and sea turtles and fish. If you are sea worthy take the Zodiacs. If you have kids or sea sick types go on catamaran. Wouldn't hurt to take bonine first.
The tubing is very low as far as thrills go. But you meander about 3 miles through caves and it is fun and you can take younger kids.
Didn't do any zip lining but there is alot of it there. Surprisingly food is not as high priced as other islands. You have to go to Bubba Burgers. There are two on island.