What to pack
 

Tip: For extra suitcase space, leave your pineapple at home!
It's always around 80 degrees, so use that as a baseline.  It rains a bit pretty much every day, and when it is cloudy and raining, it can be on the cool side.  When at high elevations, it is windy and also a little cool.  Bring at least one long-sleeve shirt.  Linen is versatile for all these conditions.

There will be some wet weather here and there.  Wettest spot on Earth, remember?  If you are going to go hiking, you will need bug spray and shoes that can get muddy.  Amphibious hiking shoes work well, but anything is fine as long as they can get muddy.  You'll have facilities for washing them at the house.  Convertible cargo pants are great for hiking Kauai.

Buy sunscreen once you get there.  There's a new law that restricts the type of sunscreen that can be used in order to protect coral.  It's about the same price as it is on the mainland.  Here's some more information and some recommendations by Consumer Reports.

Bring at least one large suitcase per family in case you want to bring anything home.

Amazon ships to the island, so if you forget something, it is often cheaper to buy it on-line and pay extra for fast shipping than to buy it on Kauai.
   

 

  

What to know

Advice worth what you paid for it!

 Pretty much everything is expensive (twice what it costs on the mainland).  Hawaii is a desirable place to live, which makes housing high, which increases the cost of everything else.  Not everything is this way, but a lot is- especially groceries.

Nature is one hell of a bargain at $0.  Be sure to take advantage of the drive to Kalalau lookout.  Investing in a pair of amphibious hiking shoes (not boots!) is worthwhile if you pan to do hiking.  Everything else costs an arm and a leg, but a lot of it is worth it, and you probably won't regret anything you do on the island.

Kauai is famous for its prolific wild chicken population, which is annoying before sunrise.  You will not be sleeping past 5:30 AM.  Either accept that and plan on going to bed early, or bring ear plugs.  We have a jar of cheap foam ones if you need some.
Where you snooze button? On head?

But that isn't much of a problem, since Kauai shuts down at 7 or 8 PM.  The wildest night life will be open until 9:00.  People go to bed early and wake up early on Kauai, so you might end up adapting to that schedule as well.
 

Don't be in a hurry.  People are slow on Kauai, whether that means restaurant/shop service or driving.  There isn't a speed limit above 50 on Kauai, and that road is the only one around the island, so expect traffic.  Slow traffic.
It takes about two hours to travel all the way around the island, so keep that in mind when planning your outings.

You are probably going to take a lot of photos.  Sign up for a free OneDrive account in case you fill up your phone or camera with photos and video.  There's a lot to see, and Kauai is photogenic.  Josh will try to code up a way for you to upload big files to his server if you run out of space, too.

"Aloha" - This greeting is overused in the tourism industry.  Among native Hawaiians, it's viewed as a very personal greeting, so it's awkward for visitors greet Hawaiians with it. So just say "hello." 
"Mahalo" - This means "thank you" and is great for everyone to use.

Be very careful swimming.  The ocean can be dangerous in Hawaii even at beaches due to currents.

The sun in Hawaii is vicious.  You will sunburn about three times as fast as Texas/Missouri.  We got burnt after 90 minutes without sunscreen in the morning, and we were already pretty tan when we arrived on the island.  Wear sunscreen.

 

What to eat

Don't eat the green part.  It's healthy.  Ick.
Always go for local specialties (like fish and tropical fruit) when you can.  Otherwise, you're just paying a lot for the same old stuff you can get at home.
Ishihara Market in Waimea - Excellent poke (pronounced "po-kay") in a family-owned grocery store!  They offer you samples before you commit to one, two, or six types.  Hawaii is where poke was invented, so this is the spot to eat it.
Olympic Cafe in Kapa'a - Looks like a kitschy, touristy beach place to get taken for a ride, but the food is actually decent and you get a lot of it. Fish was delicious.
JO2 in Kapa'a - It's upscale nouveau fusion food, but slightly less expensive as it would be on the mainland, which sort of makes it a bargain.
Tip Top in Lihue - Kind of a dump of a diner, but the pineapple pancakes are stellar.  They make a traditional Hawaiian beef stew that is supposed to be good, too.
Skinny Mike's Ice Cream in Lihue - We loved the ice cream here, and it takes a lot to impress me.

If you're going to cook at home, go by a fish market to get it super fresh.

There are farmers' markets for produce.  It's still expensive there because tourists visit these places as well, but again, it's fresh.

If you see ono (a.k.a. wahoo) on the menu, be sure to try it. It's a mild, sweet fish. In fact, "ono" is Hawaiian for "delicious."

Lilikoi is the Hawaiian name for passion fruit. Its pulp is extraordinarily sour, so don't try to eat it straight, but anything flavored with it is great!

Malasadas are one of the best things you will ever eat- if you can get them fresh. They are sort of like a doughnut, but don't do well for very long after they leave the fryer. If they aren't hot, don't bother.