Chichen Itza is an amazing place, full of history and awe. Wear sunscreen and hire a local guide for an hour; you'll enjoy the experience. Afterwards, however, be sure to wander around on your own and take the place in. Bring an umbrella, or lean against the soccer field wall for protection from any jungle drizzles. Don't forget the camera and an appetite; the restaurant on site has fantastic nachos, but don't drink the water! Save your spending money for the vendors lined up throughout the ruins, don't splurge in the gift shop.