Take a look at this site

http://www.docancun.com/mayan-ruins.htm

Chichén Itzá is most popular one. If you would like to go further, Tulum is another one, I think it is most beautiful one, also it is only ruins built by the sea. I also went to Coba, it is remote and big.

Your hotel or resort will have some package for sale, also, you can get pay taxi driver for a day to drive you to these places.