The essentials (excludes clothing items):

• Shampoo
• Soap
• Conditioner
• Facial wash
• Toothbrush
• Toothpaste
• Tissue
• Q-tips (cotton buds)
• Lotion
• Moisturizer
• Lip balm
• Towel
• Slippers
• Cap
• Shades/sun glasses
• Perfume/cologne
• Camera
• Camera charger or battery
• Cellular phone
• Phone charger
• Plastic or laundry bags - to separate dirty clothes and wet items from clean and dry ones
• Small purse - prefer one with several pockets for your celphone, loose change, hotel key, etc. Also try to get one that you can hang around your neck to avoid misplacing your stuff.
• Lighter or a box of matches - you never know when you need them!
• Bottle of water
• First Aid Kit - don't forget to include a few tablets for headache, diarrhea, and hyperacidity

Additional items for when you have plans of going swimming:

• Sun block or sun tan lotion - depending on what you want to achieve
• Swimsuit
• Goggles
• Sarong - great for the beach!
• Additional towel - you don't want to use your bathroom towel to dry you up after swimming in the beach or the public pool and vise versa!
• Beach mat
• Beach bag - prefer one with several pockets on it to separate dry from wet items

And if you're a girl, here are some other items you should not forget to include:

• Feminine Wash
• Panty liner
• Sanitary napkin - you never know when you'll need it!

Non-essentials but also good to have:

• Candies or bubble gums
• Pack of biscuits
• Small notebook and pen or your mini laptop - to record your experiences!