Chichén Itzá, the most famous site in the Mundo Maya (Maya World), was built between in A.D. 415 and 435 and was inhabited until A.D. 1204, when it was abandoned. The city is divided into two areas: Old Chichén, built between A.D. 600-900; and New Chichén, constructed in the 10th century.
The largest and most important structure is El Castillo (the Castle) with stairs ending in two large serpent heads. During the spring and fall equinox (March 21st and September 22nd) the sun casts shadows on the steps that create the illusion of a snake slithering down the face of the pyramid, if you would like to see these shadows make sure you arrive early as over 10,000 people travel here to witness this event!
Also very important are the Observatory, and the temples of the Warriors, the Thousand-Columns, the Tzompantli, the Akab Dzib, and the Jaguar. 

Opening times:
8 a.m. - 5 p.m. daily

How to get there:
Take the 180 toll road from Cancún towards Mérida for 205 kilometres (127 miles.)

Tulum. 

This impressive walled city is located on the coast and dates from the Classic period (A.D. 250-900). Its heyday, however, was during the Late Post-Classic period (A.D.1250-1540). A wall surrounded the ceremonial and political centre of the city, while the common people settled on the perimeter.
The most noteworthy structures in Tulum are El Castillo (The Castle), built on a 12 meter (39-ft.)-tall cliff facing the sea; the Temple of the Descending God, with a bas-relief carving of the god Itzamná on the facade; and the Temple of the Frescoes, elaborately painted on the inside and decorated with masks on the four cornices.

Opening times:
8:00 a.m. to 7:00 p.m. daily during the summer and from 7:00 a.m. to 6:00 p.m. during the winter. 

How to get there:
Take highway 307. Tulum is located 131 kilometres (81 miles.) south of Cancún.

Xcaret.

Xcaret opened as a park in December of 1990. Before that, and for more than a thousand years, Xcaret was a port and ceremonial centre for the Maya. It is believed the Maya would perform purification rites here, before setting sail on trading missions or crossing the bay to worship at the temple of fertility goddess Ixchel.
Today it is open to the public as an Eco-archaeological park, providing an insight to the culture of the Mayan world. There is plenty to do at Xcaret, you can swim through underground rivers, snorkel, snuba or scuba dive in the crystal clear lagoons, or just walk, enjoying the beauty of the native plants and animals that inhabit this area. Visit their website where you can download  copies of the Xcaret brochure and map in PDF format detailing all that is on offer here. Link

Opening times:
8:30 AM to 10 PM daily.

How to get there:
Approximately 1 hour south of Cancun and 5 min. south of Playa del Carmen on the Cancun-Tulum highway.

Xel-ha.

Another Eco-archaeological park, although not yet as commercialised as Xcaret, often bundled as a one day tour with the ruins at Tulum, I personally decided to give Tulum a miss on this occasion and I was glad I did. Set in around 90 acres land beside the Caribbean sea even my whole day was not long enough, and I only managed to see about a third of this tropical paradise.
As at Xcaret, the main emphasis is in and around the crystal clear waters of its lagoons and underground rivers. Again there are opportunities to snorkel and snuba as well as a chance to swim with Dolphins. I spent my first hour there just walking and enjoying the amazing scenery and wildlife.

Opening times:
8:00 AM to 6:00 PM daily

How to get there:
Take highway 307. Xel-ha is located 122 kilometres (76 miles.) south of Cancún.