Luggage and bags:
Clothing/Shoes/Weather Gear: Shorts and T-shirts and a sundress for evenings out are all that you need. Add 2 bathing suits, sandals, flipflops.
Toiletries and Medical Supplies: Must have sunscreen! You'll burn without it. Neosporin helps with coral burn and comes in individual foil packs that you can carry with you.
Camping/Beach/Outdoor Gear: Coiled strecth wrist strap for keys. Wear it in the water rather than risk someone finding your keys and breaking into your car.
Miscellaneous: I found a great printable packing list on website of the condo that we rented. It covers everything.
During the summer, Lahaina is pretty warm. Shorts, short sleeve shirt and sandals will do. From Dec.-Apr., the days are warm, but the evenings can get chilly (for Hawaiian standards around 70 F). So, a light jacket may not be a bad idea.
Camping/Beach/Outdoor Gear: Swimsuit.