Chichen Itza, which in the Mayan language means "At the mouth of the Itza well" is definitely one of the most famous sites in Mexico. Thousands of people visit this place every year, and more since it was selected as one of the new seven wonders of the world.
It is located about 200 km. away from Cancun, in the state of Yucatan. You can get there with a tour or by yourself.
We chose to go by ourselves since have never liked joining tours, we like to organize our time on our own.
It is really easy to get there if you are travelling from Cancun. Just take a bus from the ADO Bus Station in downtown for MX$240 (US$18) one way. There is only one departure per day as I remember and it is in the morning (around 8). The bus that goes back to Cancun departs at around 4 pm.
Once you get there, the archaeological site is yours to discover. There are lots of ruins to walk around and you can get some info by reading some signs by them (you can also hire a local guide outside that can explain all to you).
The park is quite big and it will probably take all morning and afternoon to see it. However, I would say the 2 main highlights of this place are the Pyramid of Kukulkan (the feathered serpent god) and the cenotes (sinkhole).
Sadly, visitors cannot climb the main pyramid anymore. It used to be possible a few years back, but because of deterioration the authorities decided that the buildings were better put out of direct access.
I heard it was a bit of a challenge to do it, but for me the biggest challenge nowadays is to avoid the massive amount of people trying to sell you stuff.
Don't get me wrong, they are all trying to make a living, but probably by the time you are getting ready to leave, you are kinda tired of walking under the sun and being bugged by lots of vendors adds a bit to the exhaustion.
Overall, a very interesting place!