If you're into "seeing" for the sake of seeing, you can do Valle de los Caidos, Avila, and Segovia in a day (a long day...with Segovia in the evening). Having been to all three, I recommend Avila and Segovia for sure. VdlC is okay, but don't take too long (you won't have to worry...not much there).

The cathedral in Avila is built into the walls, so you can kill two birds with one visit there...go see cuatro postes on the lower part of town, and head out to Segovia.

Many have expressed their opinions already on Segovia. Do a thread search and see what you think. My top five: Cathedral, aqueduct, Alcázar, Romanesque churches in town (San Miguel...) and La Iglesia de la Vera Cruz, located below the Alcázar on the road towards Zumarramala.

Have a great trip!