A helicopter flight around the island is definitely a must, and one of the few ways to see certain areas like the Na Pali Coast and the stunning waterfalls inland. There are tours that will take you around the island and show you the sights where numorous movies were filmed.
You must do a Luau. There is an abundance of Feral cats on the island... I befriended four of them and even adopted one and brought him home!
Quick Tips:
Ask the locals where to grocery shop or you'll be in for a big shock! Food costs are much higher there and stopping at a 7 - 11 for milk will set you back .18 ***ouch***. If you shop at Saveway, you can get a preferred card, which will save you some. I LOVED the shopping at the local shops and tourist areas. The Cocoanut Mall in Kapa'a is the best known and largest tourist shopping shop on the island. If you're really budget minded, you can go to Walmart in Lihue and find many of the same souvenirs MUCH cheaper.
Best Way To Get Around:
Renting a car is the only way to really enjoy the island. The scenic overlooks offer beautiful views for picture taking. To view the Na Pali coastal area, expect to be taking a helicopter ride or scenic boat ride, as it's the only way to see the west coast. The roads do not go all the way around the island due to the gorgeous Na Pali coastal cliffs.