Seeing Kaua'i from the air is highly recommended. We took a small plane ride, which was great. Use AirVentures.
Driving up the Waimea Canyon (also known as the Grand Canyon of the Pacific)
Sunset cruise
Walk along the beaches
Drive up to the North Shore
Polihale Beach
Quick Tips:
Use The Ultimate Kaua'i book. It’s worth picking up before you go. It is sold on the island, but look through it before you get there. We showed it to some locals that we met on a hike, and they didn't even know some of the places described.
There’s a discount website for Kaua'i activities: . We didn't have any problems with anything that we booked through them and saved money at the same time. One of the activities was cancelled, and they refunded the deposit immediately, no problems.
Best Way To Get Around:
Rent a car unless you want to sit on the beach the whole time. There is a lot to see on Kaua'i, and it is best to rent a car. Car rentals seemed cheaper than on Oahu. We booked through Hertz. I kept checking the rates as our trip got closer. I saved about from when I first booked the car.