You're going to a small town that borders the beach on one side and the jungle on the other. The weather is generally hot and humid. Even in the evenings, when it cools down slightly, it's still warm. Rain showers are also common, especially during the rainy season. Bring sandals, bathing suits, light clothing, sunglasses, sunscreen, and anything else you'd generally pack for the beach. It's also smart to bring comfortable shoes that are safe for walking and/or hiking if you choose to do more adventurous activities. I've never put on a pair of heels in my life in Tulum; even though there are nice restaurants, attire is "beach chic" at most, and heels/wedges aren't practical when trying to walk on sand or down the beach road. Save the space in your suitcase for an extra bathing suit or for anything you want to bring back from Mexico with you.
It's smart to pack bug repellent (you are in the jungle, after all), paper copies of your passport and any hotel/transportation information you may need in case you don't have cell service or WiFi, and a small flashlight for beach walks and in case of power outages (hey, they happen from time to time). Skip bringing a blow dryer — let that salty ocean water and sea breeze do its own magic.
