-Clothing. Roatan is hot most of the year, so just shorts and t-shirts or tank tops (or sundresses) is usually fine, though the evenings can cool off a bit during the winter months, so you might want a light sweater or jacket. October to January is our rainy season, so bring a pair or two of long pants, sweatshirt, and rain jacket/umbrella. A down parka is never needed, you can leave that at home :) A bathing suit (or several) is a must, and a sarong is about the most versatile thing you can pack- it's a skirt, it's a towel, it's a blanket!
-Footwear. Sandals are probably the only thing you'll need, running shoes if you like to go jogging. No need for fancy shoes, Roatan is not a dress-up kind of place. There are spots to go dancing, but everything is pretty casual, not worth taking up luggage space for more shoes. In the rainy season, rubber boots are great to have, but they're pretty heavy to pack, so unless you're just coming to Roatan, I'd stick with the sandals and just rinse off your feet when they get muddy.
-Toiletries. Other than your basics of shampoo, deodorant, toothpaste, other Roatan essentials include sunscreen, bugspray and after-bite treatment. Don't worry, everything's available here, if you don't have something along with you.
-Towel. If your'e bringing a light-weight travel towel, you'll also need something to lie on at the beach (refer to clothing and the sarong suggestion). You can rent towels at the hostel for $1- that saves a lot of space in your pack, and then you don't have to be carrying a wet towel when you leave.
-Flashlight. Is it needed? Well, a small travel flashlight is great to have. A lot of the roads aren't well lit here, and the power goes out occasionally. Plus, if you're sharing a dorm room and want to do some late-night reading in your bed, a flashlight is the way to go.
-Electronics. Bringing along a smart phone or tablet? Nowadays, people don't go far without some kind of electronic. If your phone is unblocked and you're staying awhile, you can buy a local SIM chip. Keep in mind that there is a higher rate of thefts in Honduras than elsewhere in the world, so if it's expensive, include it on your insurance, and don't leave it unattended (in your bag on the beach, for example).
-Money, credit card. How much to bring? Again with the higher theft rate, you don't want to be walking around with wads of cash, but there have been on-going problems with tampered ATMs if you're planning on doing withdrawals (you probably want to stick with ones that are at a physical bank, and monitor your account for fraudulent activity). If you're bringing a credit card, make sure to inform your credit card company that you'll be traveling and where, so that they don't block your card.
-Spices. Like to cook? I saw a traveler that had a pill container (the one with a little section for each day of the week), that they used to travel with several different spices, so they wouldn't have to keep buying them or traveling with big bottles. I think that's a great idea. There is a communal kitchen at the hostel, and there are things that other travelers have left behind that you're welcome to use, but you never know what will be there.
-Passport. It's a good idea to take a photo of it and email it to yourself, so you have a copy of you should lose it.
-Positive attitude. Trips often take turns that you weren't expecting, and sometimes things don't work out as planned. Lost your luggage? Bag got dropped in a puddle? Dive class got cancelled? Friend that you're traveling with fell in love and decided not to move on to the next stop with you? Just have the mind-set that there is always something to learn and discover, and make the most of each day, regardless of whether it's what you had anticipated.