Valladolid's 16th-century cathedral is not Castilla's finest, but it does have an extravagant altarpiece by Juan de Juní and a processional monstrance by Juan de Arfe in the attached Museo Diocesano y Catedralicio. Guided tours (adult/concession €5/4) of the cathedral and bell tower last 45 minutes and the views are fabulous; combined with a guided visit to the museum (adult/concession €8/6) it takes two hours. (You can visit the cathedral and museum on your own, but not the tower.)
The 13th-century ruins of the Collegiate Church, atop which the cathedral was built, can be seen along the cathedral's northeastern perimeter.