The archeological site you must visit is "Chichén Itzá" is considered to be one of the new seven wonders of the world; to complete the entire excursion you can take a lunch & cenote snorkeling stop.
A cenote is a deep water-filled sinkhole in limestone that is created when the roof of an underground cavern collapses. This creates a natural pool which is then filled by rain and underground rivers.
And lets visit Valladolid, You can take a walk around their beautiful plaza surrounded by small souvenirs shops that sells from T-shirts, Silver, Local Clothing, Zarapes and much more. Or you just sit and enjoy a soft drink or eat a marquesita.
Marquesita - A crepe is rolled around shredded edam cheese and condensed milk. The crepe hardens after rolling and reminds many of an ice cream cone. Alternative fillings such as nutella may also be offered.
These an much more is offered on all the different excursions you can chose.