pretty much what we did 15 years ago. Don't know how much is changed since then.
Kauai is your chance to be active. Bike tours down the mountain, snorkeling on the north side, kayaking, hikes, etc... It's all good.
If the weather sucks - get up and drive 15-20 minutes and find good weather.
There was a beach called the "secret beach" on the north side that was a PIA to find and walk to, but totally worth it. Since it is so out of the way, dress can be VERY casual. cough, cough.
Eat where the locals eat.
We had a nice helicopter ride.
Have a game plan plan before you go so you feel in control of your time there, but don't give a 2nd thought to blowing something off.