Yes, Santo Domingo de Silos. Two nights gives you the opportunity to hear the monks chant the entire Liturgy of the Hours, if you are so inclined, or you could go to morning and/or evening prayer, then visit the nearby film site where Clint Eastwood and his enemies shot it out in "The Good, The Bad, and The Ugly."
Leon is definitely worth some time. Tour the Catherdal, which is magnificent, and the adjoining museum, as well as the museum at San Ildefonso, a few blocks away, where some of the kings of Spain are buried. And enjoy the passing scene on the Plaza with some tapas and vino tinto.
The Cathedral in Burgos is another gem. And it contains the tomb of El Cid. I'll have to check the locations, but on the main street just before the cathedral is an imposing statue of The CID, and just beyond that a bank housed in a building that was once a palace of Isabel and Ferdinand. It's where Christopher Columbus presented his report on his second voyage to the new world. There's a plaque outside to mark it.
And, of course, there's always the fun of stopping early in a couple little villages that appeal to you and being part of the local scene.
Enjoy!