When we went in March we went to sign up for the ride on tubes thru the tunnels, but were told at that point they are booked 3 weeks in advance.

Also for the helicopter ride - if you reserve in advance you can get their discounted fares. We did the Blue Hawaiian helicopter.

Definitely get the 'blue bible' - "Kauai Revealed" by Andrew Doughty and Harriett Friedman
http://www.hawaiirevealed.com/kauai-revealed/