Here's a list of great stuff to do in Kauai.

Inner tubing at a sugar plantation

This is a blast! Run by Kauai Backcountry Adventures, it's the only inner-tubing activity on the island. Very well organized and well run. The inner-tubing ride is about an hour, and you go through open areas as well as cave-like tunnels. See Trip Advisor reviews on this for more details.

Waimea Canyon Drive and Koke'e State Park

Stop and take pictures on your way up to the canyon and at the top. Hike the trails. Enjoy the fabulous scenery!

  Kipu Ranch ATV

Lots of  "clean" dirty fun! It's actually a little bit of a workout, because you have to sometimes muscle your ATV through muddy sections.  Kipu is a good operator because they don't have governors on their ATVs, and they take you to see Kipu Kai, which is a privately-owned, beautiful area of Kaua'i that even most residents have never seen. Lots of great trails, and the guide tells you about different movies filmed on the ranch, and some Hawaiian folklore.

Kilauea Lighthouse

Really cool, fantastic views, totally worth the couple of bucks that the Fish and Wildlife folks charge for admission (money goes to support the seabird sanctuary there).  Don't miss this. In the visitors center, binoculars are available to use for free. If you're lucky, some seabirds will be nesting there and you can see the little chicks. Park naturalists are friendly and enthusiastic about the birds, and happily answer questions.

Kaua'i Museum

A neat little museum that's not to be missed. Not really for little kids; more of an adult thing, although older kids and teens would probably enjoy it. Morning admission includes an optional 90-minute guided tour. TThe tour guides are great storytellers. They teach you a lot of interesting history and facts about Kauai that will stick with you. Even if  you don't go on a tour, there's neat exhibits of Ni'ihau shell leis, Philippine people's contribution to Kaua'i, early Polynesian settlers, Captain Cook's adventures, and much more. The museum is the perfect size -- just enough to learn something without feeling overwhelmed. Do-able in one morning. By the way, you can get 2-for-1 admission coupons in some of the tourist booklets.

Inter-Island Helicopter ride

These guys are military pilots, and they fly with the helicopter doors OFF. Very exciting and great for picture-taking. Really phenomenal and worth the money for the trip. They have a one-hour ride that's about $200 per person, and a longer trip where they land at a secret waterfall that's a bit more per person. Incidentally, Inter-Island Helicopters handles the search-and-rescue missions on the island. They're based out of Hanapepe, on the island's west side, so if you take a morning helicopter ride, you can go to Waimea Canyon in the afternoon.

Snorkeling at Poipu Beach Park

Especially great on summer mornings. Lots of different fish and the water's pretty clear.

Friday Art Night in Hanapepe

This is the town that "Lilo and Stitch" creators modeled the movie's town on. Friday art night is not for kids, but is a wonderful evening out for adults and possibly teens. All the galleries and stores are open from 6-9 pm and there's live music, people out and about, and some of the galleries offer free cookies, nuts, and other munchies. A fun way to get acquainted with the artists on the Garden Isle and their work. The Hanapepe Cafe is open for dinner only on Friday nights, and we heard the food was very good there.

Kaua'i Coffee Company

A little stop-over jaunt on the way to and from the west side -- not really a destination in itself, but worth checking out. There's a gift shop, a 20-minute self-guided walk, a video, and free coffee tastings. If you're not into coffee, then you'll probably want to skip this. But if you like coffee, this is kinda neat.  

 Zipline-ing at Princeville Ranch Adventures

After a short safety briefing, try your hand at Ziplining on this working cattle ranch.  There is a zip and dip trip that includes 6 or 7 ziplines and a lunchtime stop at a small lagoon with cliff diving optional.  Even folks that are afraid of heights can have a wonderful time, and the guides are pleasant to work with.  Take some sturdy shoes as this is not the spot for flip flops.  Don't forget your digital camera!

 Sail the Napali Coast with Capt. Andy's Charters

With an abundance of wonderful scenery that has been featured in lots of movies, the Napali coast is one of the most breathtaking places in Hawaii.  One way to see it is a catamaran tour.  Many vendors out of Port Allen offer Catamaran tours that normally include Lunch and snorkeling.  Captain Andy's is one of these vendors, and the tour takes about 4-5 hours on their cat(s).  The scenic backdrops and wide variety of wildlife encountered will leave you breathless!  It is not uncommon to see whales, dolphins, honu (turtles), and a variety of birds and reef fish (assuming you snorkel) on these trips.  Most of the trips include snorkel use and the gear was top notch.  Some of this area can only be seen from air or boat, unless you are willing to hike 6-8 hours to get there.