Even before airlines started charging for checked bags, Hawaii was a good place to cram everything in a carry-on. The following assumes you are an outdoor type planning to hike and swim during a two-week stay.
Flip flops (called slippers in Hawaii) are the go-to shoe that locals spend 90 percent of their time in, when not barefoot. Get a high-end model with a good foot bed (Teva makes probably the best). Note: The sandal type strap on shoes (Keen, Teva, etc.) can be a problem when used at the beach. Once they get wet, sand gets in and tears feet to shreds.) Great selections and variety are here; wait until you arrive to buy.
You'll want a lace-up low cut for hiking and the plane. These can double as a dress-up shoe, though you will very likely get them muddy. Don't worry about Gore-tex, since you will dunk the shoe entirely on stream crossings.
Hawaii is not a dress-up place. For fancy restaurants, men may wear long pants and women skirts, but not necessary. Make sure to get "dri-fit" synthetic tops: they are lightweight, breathable, and dry fast after rain or in-room washings. Bring both long and short sleeves.
A retractable hiking pole is very useful on mountain trails that tend to be steep and slick.
Swim fins and a mask can be easily purchased in Hawaii, cheaply, at a Wal-Mart or Long's Drugs. It's also easy to buy board shorts (men's swimsuit), swimsuits, and rash guards (an optional top worm for snorkeling and surfing).
A Gore-tex lightweight parka or shell is very necessary for hikes of any length, especially the higher elevations (Haleakala on Maui; Mauna Loa and Mauna Kea, Volcanoes National Park on the Big Island; and Waimea Canyon on Kauai).
Hat and sunglasses, of course, are a must.
Bring a lightweight daypack (your second carry-on) and get organized on day one. Include Band Aids and anti-biotic oinment. Cuts need to be washed and treated right away. Avoid exposing cuts to freshwater. Women and men alike can use their pack as a "purse" when getting in and out of the car, as well as on the trail. Always have extra food and water (never drink stream water), as well as the usuals, like Swiss Army knife, LED headlamp, Ibuprofen, sunblock, lip balm. Leaving valuables in the car is not recommended, especially on Oahu. And too often, you may find yourself going farther than you initially planned, so get in the habit of hauling the pack.