People ask me this all the time. Often they are cruise ship visitors with only 8-24 hours to "see" the island. And though I can easily prove that a day is not enough to see the island, it is enough to be become smitten with it.
Kauai is different than every other island in that it is lush and looks like that a tropical island is supposed to look like--if you know where to go. Save sunny beaches for Maui ahd Oahu. In Kauai you get to experience the garden isle in all its splendor.
Start by renting a car through your cruise ship or at the Lihue airport (you can take a cab from the harbor). Every major car rental agency is located at the airport.
After you have your car, drive north to Princeville (if you are starving, pitstop at Mango Mama's for a fruit smoothie). Head to the Hanalei Bay Resort's Bali Hai restaurant to have breakfast. Though not the best service, the views remind you why South Pacific was filmed on the island.
Keep traveling north, through the town of Hanalei (you'll be back) to the Limahuli Garden for a tour of native plants. Then drive north until the road ends to stop at the picturesque Kee Beach.
Venture back through Hanalei, stopping at the surf shops and art galleries.
When your belly rumbles, head straight to the Kilauea Fish Company for fresh fish wraps and veggie offerings. Even better take your food to the Kilauea Lighthouse for a picnic with views of the albatross sailing over the sea.
If you want a little beach break, drive back to Wailua to enjoy the kid-friendly Lydgate Park.
Or head ot old town Kapaa for some souvenir shopping and a cocktail.
For dinner, I love Hukilau Lanai for fresh (and super sweet) seafood and decadent cocktails.
By that point, you should be tuckered out and ready to return to Kauai for a month.
Pick up a copy of GREAT DESTINATIONS KAUAI (Countryman Press) for more information.