These are just a few of the places that I have visited down the years and which I would definitely recommend. I would say that one or two of them (like Salamanca, Cuenca and even Avila) may warrant an overnight stay due to their distance. However, that shouldn’t be a bad thing as they have some beautiful hotels and/or paradors.

Segovia
– UNESCO World Heritage Site. Home of the famous Roman aqueduct, Alcázar and Cathedral. A wonderful place to walk around, with some great restaurants and bars.
Toledo
– UNESCO World Heritage Site. Home of the painter El Greco and famous for its swords and for the mix of Moor, Christian and Jews who have lived there down the ages and who have all left their mark on this fabulous city. Look out for the Cathedral, Greco’s House and the Alcázar.
Avila
– UNESCO World Heritage Site. Reputed to be the longest walled city in Europe, Avila is a city famed for being the brithplace of Saint Teresa de Jesús and is a treasure trove of churches and palaces. It is the capital of Castilla-León and is famed fr its T-bone steaks and Yema de Santa Teresa (sweet cakes).
Aranjuez
– Declared an UNESCO World Heritage Site in 2001, Aranjuez is famous for its wonderful Royal Palace and for its beautiful gardens which sit next to the river Tagus (Tago). 
Cuenca – Guess what? Another UNESCO World Heritage Site and famed for its Hanging Houses (Casas Colgadas) the Enchanted City (Ciudad Encantada) on its outskirts and beautful town centre . One of the Hanging Houses actually houses the Spanish Museum of Abstract Art.
Alcalá de Henares
– This city, to the East of Madrid, is famed as being the birthplace of Cervantes who wrote Don Quijote. It also has one of the most important unviersities in Spain and a number of important buildings and churches, not to mention its historic quarter. And by the way, it’s another UNESCO World Heritage Site!
Salamanca
– Considered to be one of the most spectacular Renaissance cities, Salamanca is renowned for its Univeristy, which is reputed to be one of the oldest in the world. The city has a beautiful Plaza Mayor, which many consider to be equal to or better than Madrid’s.
El Escorial
– El Escorial is yet another UNESCO World Heritage site and is famed for its huge Royal Monastery. It was built by Phillip II and was designed to be the political centre of his empire. Aside from the monastery, museum, palace and library you will find the Royal Pantheon which houses the tombs of virtually every Spanish king since Phillip II’s time and numerous priceless paintings. Nearby you have the Valley of the Fallen (Valle de los Caidos) where the Fascist Dictator Franco is buried – it is a rather disturbing, yet impressive monument – a church built into a mountain.