For sightseeing, casual lightweight clothing and good walking shoes are appropriate; most restaurants don't require very formal attire. For beach vacations, you'll need lightweight sportswear, a bathing suit, a sun hat, and lots of sunscreen. A sarong or a light cotton blanket makes a handy beach towel, picnic blanket, and cushion for hard seats, among other things.
Travel in forest areas will require long-sleeve shirts, long pants, socks, sneakers and/or hiking boots, a hat, a light waterproof jacket, a bathing suit, and plenty of insect repellent. Other useful items include a screw-top water container that you can fill with bottled water, a money pouch, a travel flashlight and extra batteries, a Swiss Army knife with a bottle opener, a medical kit (with first-aid supplies and basic over-the-counter remedies), and binoculars. If you have the room, try to take any clothes you don't wear as well as extra soap, over-the-counter medicines, and so on, to give away. That bottle of aspirin you bought at a discount drugstore back home could be a godsend for a retired school teacher trying to survive on CUC$10 a month; the blouse that no longer fits might be the perfect gift for the housekeeper at your hotel.