Pack light. That means washable, light, summer clothing. However, it also means don't pack too much. Leave room in your suitcase for clothing purchases. Even evening events call for casual attire. Bring a few pairs of shorts, T-shirts or light tops, a couple of pairs of long pants and a waterproof windbreaker or raincoat. It may be good to add a sweatshirt to your list. You may experience a cool evening when you will need something warm. Don't forget the basics of underwear, socks, sun hat, bathing suit, cover up and a beach towel.
You will only need three pairs of shoes, at the most. A pair of walking shoes is a must for hiking, shopping and just touring around. A pair of flip-flops will come in handy on the beach and lounging poolside. Bring a nicer pair of sandals for night time entertainment and finer dining.
Bringing your own insect repellent and sunscreen will save you time, although it can be purchased in most tourist towns. Take your passport and two copies of the passport page that has your picture on it. A passport is not required, however. You can instead bring along your driver's license and birth certificate, with copies. Pack the copies in an alternate location, in case a bag gets lost. If you plan on doing quite a bit of hiking, a light backpack is in order. Most places in Mexico accept American currency. It's best to take half your money in cash and the other half in traveler's checks. A money belt is handy to keep your cash and passport safe. There are, of course, banks and money exchange booths if you need Mexican currency. Sunglasses, watch, hand sanitizer and toiletries will come in handy. Don't stress out about your packing too much. Most things you forget to pack can easily be purchased on your trip.
