Phone and charger. Do you have international coverage? Watch out for roaming charges!
Breathable clothes. It can get extremely hot and humid in Central America. I find I rarely want to wear jeans, though many of the locals do. Lightweight, loose pants are great. So are long skirts and dresses. Bring a thin long-sleeved shirt for balmy evenings when the mosquitoes are out. How conservative you dress depends on the area you're traveling through; for example, beach areas are much more casual than cities or Mayan villages.
A swimsuit. Or two!
A jacket or windbreaker. Preferably waterproof. How heavy depends on what region you'll be traveling through; the Belizean islands in summer differ from the Guatemala highlands in rainy season, for example.
Hiking shoes. These can be boots, sneakers with grip, or Tevas, depending on the level of trails you intend to tackle.
Sandals or flip-flops. For the beach, for the evening, for shared bathrooms.
A jacket or windbreaker. Preferably waterproof. How heavy depends on what region you'll be traveling through; the Belizean islands in summer differ from the Guatemala highlands in rainy season, for example.A towel. This is up to you, but I like to bring a thin beach towel when I backpack. I know travelers who are in love with their synthetic chamois travel towels.
Rainy weather supplies, like a travel umbrella and a cheap poncho. Even if you're not traveling in Central America's rainy season, you should anticipate some showers.
Glasses or contacts. Make sure you bring extras.
Sunblock. Central America is close to the equator, and the sun can be harsh. Look for waterproof sunscreen, with a sun protection factor (SPF) over 30.
Prescription medications. Are you taking any? Make sure you bring enough.
Other medicine. I wish I didn't have to, but I recommend bringing both Pepto-Bismol chewable tabs and Imodium. You're exposed to a lot when you travel, even if you're careful. Even better, ask your doctor for a prescription for antidiarrheal medicine with the antibiotic Cipro. You shouldn't use it unless your stomach is extremely upset (stick with the OTC stuff first), but it's great to have around.
Hand sanitizer. Use often, especially before meals.
Toiletries. Toothpaste, laundry soap, makeup, shampoo and so forth are all readily available in Central America, so if you forget something you'll be fine. However, it's less likely your favorite brands will be stocked, and will likely be marked up if you find them.
DEET bug spray. Malaria, dengue fever and yellow fever are all mosquito-borne, and besides, mosquito bites suck! (Lame pun intended.) You can also find DEET insect repellent wipes, which work well in a carry-on. I've found it's pretty expensive to buy in Central America, so bring enough.
Travel vaccinations and malaria pills. Make sure you investigate which travel immunizations are recommended for your Central America destinations, and whether or not the CDC recommends you take malaria pills.
Debit card(s), cash and/or traveler's checks. Some Central America travelers still like to carry traveler's checks, but they're becoming more obsolete as ATM machines crop up all over the isthmus. I usually bring a decent amount of cash -- most countries accept American dollars (the official currency in Panama and El Salvador) -- and store it in several places, along with a debit card and credit card. Make sure you have the emergency number for your bank.
Health insurance. Are you covered abroad? If not, buy some travel insurance.
A book. Or two. Because there's always unexpected down time.
Also, take a look at our other packing tips for Central America travel: