Hawaii is casual: sandals, bathing suits, and comfortable, informal clothing are the norm. In summer, synthetic slacks and shirts, although easy to care for, can be uncomfortably warm.
Probably the most important thing to tuck into your suitcase is sunscreen. This is the tropics, and the ultraviolet rays are powerful, even on overcast days. Doctors advise putting on sunscreen when you get up in the morning, whether it's cloudy or sunny. Don't forget to reapply sunscreen periodically during the day, since perspiration can wash it away. Consider using sunscreens with a sun protection factor (SPF) of 15 or higher. There are many tanning oils on the market in Hawaii, including coconut and kukui (the nut from a local tree) oils, but they can cause severe burns. Too many Hawaiian vacations have been spoiled by sunburn and even sun poisoning. 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, and don't forget to wear it. All major hotels in Hawaii 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 for a couple of dollars and off-brand sandals for $20. 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 or planning to visit a high-altitude area, bring a sweater or light- to medium-weight jacket. A polar fleece pullover is ideal, and makes a great impromptu pillow.