Maui is very informal: You'll get by with shorts, T-shirts, and sneakers at most attractions and restaurants; a casual sundress or a polo shirt and khakis is fine even in the most expensive places. Dinner jackets for men are required only at very few resorts, such as the Lodge at Koele on Lanai, and they'll cordially provide a jacket if you don't bring your own. Don't forget a long-sleeved cover-up (to throw on at the beach when you've had enough sun for the day), rubber water shoes or flip-flops, and hiking shoes and several pairs of good socks if you plan to do any hiking. You might also want to bring binoculars for whale-watching.
Be sure to bring sun protection: sunglasses, strong sunscreen, a light hat, and a canteen or water bottle if you'll be hiking -- you'll easily dehydrate on the trail in the tropic heat. Campers should bring water-purification tablets or devices.
Don't bother overstuffing your suitcase with 2 whole weeks' worth of clothes: Maui has laundry facilities almost everywhere (except Hana). If your accommodations don't have a washer and dryer or laundry service (most do), there will most likely be a laundry nearby.
One last thing: It really can get cold on Maui. If you plan to see the sunrise from the top of Haleakala, bring a warm jacket; even in summer -- when it's 80°F (27°C) at the beach, 40°F (4°C) upcountry temperatures are not uncommon. It's always a good idea to bring long pants and a windbreaker, sweater, or light jacket. And be sure to bring along rain gear if you'll be in Maui any time from November to March.
