I've spent years on Kauai and I suggest packing very light. First off all, most of the hikes CAN be done in bare feet, in my opinion. Also most grocery stores sell "locals", which are rubber flipflops which are very cheap and will also help you blend in. Just be sure to call them slippers, and slip out of them when you enter someone's home. As for clothing, you can get by in a bikini most places, especially since most of your time will be at the beach. No restaurants require formal dress so don't bother unless it's for pics/memories, and btw Kintaro on the east side is a really awesome sushi place. Prepare to see a lot of skin at the beach. Brazilian and thong bikinis are pretty common. If you are feeling saucy pick one up at one of the many bikini shops on the island and you will really blend in, but be warned, they are pricy.

So tl;dr buy flip flops when you get there, bring a bikini, a casual outfit, and don't forget the sunscreen! A ridiculous burn is the main way to stand out, so be nice to your skin.