Organized Tour of Valle de los Caidos and El Escorial
Seeing everything El Escorial and the Valle de los Caidos have to offer in a single day is going to be hard work. Miss the bus to the valley and you won't get to see anything. Miss the bus back and, well, make sure you brought your walking shoes.
You have two choices - stay in El Escorial for two days (not recommended, there isn't that much to see), or take a guided tour from Madrid. The below tour takes you from Madrid to El Escorial and on to the Valle de los Caidos all in one day.