Valladolid is a quiet colonial city and the perfect base camp to explore Chichen Itza and other Mayan ruins, like Ek Balam. There also nice cenotes in the area.
In Chichen Itza you will find the most impressive pyramids but it is also extremely touristic. So get there early, before the massive buses from the Cancun resorts arrive! I find amusing that some tourists, despite being surrounded by hundreds of people, decent toilets and comfortable restaurants, insist to dress as Indiana Jones.