Kauaʻi is casual, warm, and wetter than people expect, so packing is less about quantity and more about the right few things. You can buy almost anything you forget once you are here, but a handful of items are worth bringing from home. Here is what actually earns space in the bag.
Mineral (reef-safe) sunscreen with zinc oxide or titanium dioxide, because the chemical kind is banned for sale here and the sun is strong. Water shoes or sturdy sandals for rocky entries and trail walking. A light rain jacket or packable shell, since the north shore in particular gets passing showers year round. A reusable water bottle, because tap water is excellent and you will drink a lot of it. And a dry bag or zip-top bags to keep your phone and keys safe at the beach.
Kauaʻi's weather changes by region more than by day. The south shore around Poʻipū is the dry, sunny side, averaging roughly 30 to 40 inches of rain a year. The north shore around Princeville and Hanalei is greener and wetter, roughly 75 to 85 inches a year, mostly in winter. Mount Waiʻaleʻale in the center is one of the rainiest spots on Earth at around 450 inches a year, which is why the island is so lush. Practical takeaway: bring sun gear and a rain layer, and if one side is grey, drive to the other side.
Dress is relaxed everywhere, including most dinners out. Bring quick-dry clothes, two swimsuits so one can always be drying, and a light long-sleeve layer for cool evenings and air conditioning. For shoes, you want sandals plus one pair of real trail shoes if you plan to hike. Trails like the ones near Waimea Canyon or the Kalalau Trail get muddy and slick, and flip-flops will not cut it.
A good reusable snorkel mask if you have one you like, since rental and store masks are hit or miss. Reef-safe bug spray for shady or rainy hikes. A portable phone charger for long beach days. Motion-sickness remedies if you are doing a Na Pali boat tour or a helicopter flight. And a small first-aid kit with reef-cut supplies, because coral scrapes are common and slow to heal.
Chemical sunscreen with oxybenzone or octinoxate, which is banned for sale and not allowed on the reef. Heavy formal clothes, which you will not use. Big beach umbrellas and chairs if you are flying carry-on only, since you can rent or buy cheap ones here. And do not overpack toiletries, since there are stores in every main town.