Hawaii is casual: sandals, bathing suits, and comfortable informal clothing are the norm. Year-round, clothing of cotton or rayon proves very comfortable. Local women love to wear the pareau, or sarong. For men, you'll look right at home in T-shirts and board shorts.
One of the most important things to tuck in your suitcase is sunscreen. Recent research has indicated that some traditional sunscreens are harming coral reefs; indeed there's legislation currently being introduced to ban the sale of these products in the Islands. So if you do want to use sunscreen, buy products that are zinc-based or reef approved. Even better? Buy a long-sleeved rash guard, available at all major retailers. No gunky lotions or harmful chemicals to deal with while out enjoying the reefs. Hats and sunglasses offer important sun protection, too. Both are easy to find in island shops, but if you already have a favorite packable hat or sun visor, bring it with you. All major hotels in Hawaii (and most small ones) provide beach towels.
As for clothing in the Hawaiian Islands, there's a saying that when a man wears a suit during the day, he's either going for a loan or he's a lawyer trying a case. Only a few upscale restaurants require a jacket for dinner. The aloha shirt is accepted dress in Hawaii for business and most social occasions. Shorts are acceptable daytime attire, along with a T-shirt or polo shirt. There's no need to buy expensive sandals on the mainland—here you can get flip-flops (called "slippers" by locals) for under $5. Golfers should remember that many courses have dress codes requiring a collared shirt; call courses you're interested in for details. If you're not prepared, you can pick up appropriate clothing at resort pro shops. If you're visiting in winter, bring a sweater or light- to medium-weight jacket. A polar fleece pullover is ideal, and makes a great impromptu pillow.
If your vacation plans include Hilo, you'll want to pack a compact umbrella and a light poncho. And if you'll be exploring Hawaii Volcanoes National Park, make sure you pack appropriately as weather ranges from hot and dry along the shore to chilly, foggy, and rainy at the 4,000-foot summit. Sturdy boots are recommended if you'll be hiking or camping in the park.