Valley of the Fallen — worth it?

Nah... you will use your time much better skipping the Francoist regime monument at the Valley of the Fallen, I would suggest better the UNESCO heritage medieval cliff-side town of Cuenca, about 1 hr west via the AVE.

When we visited it we had a rental car, so getting to it was easy. We first went to San Lorenzo de El Escorial to see the Royal Seat of San Lorenzo de El Escorial, then El Valle de Los Caídos. It is only 14km from El Escorial. I enjoyed visiting it. Now that Franco’s body has been removed from it and moved to another location, hopefully it isn’t as politically sensitive as it was. Not being from Spain, rather than looking at it as a Francoist regime monument we looked at it from the perspective of being a monument to everybody who died during the Spanish civil war. There are more than 33 thousand bodies in its crypts. It never escaped us that it was built using convict labor, but it is still an impressive accomplishment. When coming from Madrid on the A6, the massive 150-meter high cross can be seen from miles away. The view from the monument looking east is spectacular. Believe we spent less than 1.5 hours there.

Carlos’ suggestion of Cuenca is a good one.

At the risk of being a little political here, I am with Carlos and I do not think that the recent "cleanup" is enough to hide the original purpose of the monument, which is mainly to glorify the dictatorship which commissioned it. Franco isn't there anymore, but his inspiration Primo de Rivera is still buried there, after all.

Thus, I would pass for now. A very personal decision of course, certainly influenced by the fact that half of my grandparents fled the Spanish Civil War.

El Escorial is easily accessed by train and would already fill most of the day anyway.

Skip the Valley of the Fallen. I certainly would not rent a car just to get there and/or to El Escorial.
El Escorial is ok, but I'd suggest skipping that too. If you do decide to go to El Escorial you can get there by train. Other daytrip options seam much more appealing. Have you been to Avila? La Granja de San Ildefonso? Aranjuez? Alcalá de Henares? Chinchón? Even places like Sigüenza - there's a train that gets you there in an hour and a half so it could be doable as a daytrip.

There are certainly no lack of day trips from Madrid, as mentioned above.

While not traditionally considered a "Madrid day trip", I'd make the case that even Zaragoza can be a practical day trip from Madrid. Zaragoza is only about 1:15 hr on the AVE high speed line from Madrid Atocha (most zoom past it on their way to Barcelona).

It's just a 15 min walk from Zaragoza's train station to the Aljafería Moorish Palace, and then it's only another 15 min or so walk to the Casco Antiguo (old town) and Basílica del Pilar.

Also but not yet mentioned in this thread: Salamanca.

Thanks all!

I think I'll stick with the plan to take the train to El Escorial. Some timing dictates a quicker day trip than the further-afield options would allow.

I understand the challenging history and politics around the monument, but would still like to visit it someday. Society can't learn from history it ignores.

Of course, of course! The conversion from monument to memorial has barely started (with the removal of Franco's remains). If and when it makes progress, then I would be willing to visit too!
Just not yet.

Agree with balso, unfortunately the Valley of the Fallen does not represent history, but a manipulation of history in a feeble attempt at "reconciliation", most can see through this façade. I guess a similar example in the USA would be Confederate Monuments.

If you are interested in learning about our Civil War, know that the Aragon region was a pivotal battleground during much of the Civil War. The capital of the region, Zaragoza, is a good base for a few days just to explore the city itself and then make daytrips out to Civil War sites like Belchite (a better memorial to the war). There is a Spanish Civil War Interpretation Centre just north of Zaragoza, in the village of Robres.

In the future, you may find interesting to actually follow the George Orwell route in the Aragon region. On the route you will find the very places that inspired his writings on the Civil War, you can read more about the route here: https://www.spain.info/en/discover-spain/george-orwell-route-aragon/

Valley of the Fallen is not essential, but I don't agree that a visit is a mindless vote for Franco. The controversy over moving his body, and the (original) decision to bury combatants (behind the walls) together are thought-provoking. BTW, we found a (scorned by many on this forum) tourist bus outing to be very convenient, efficient, and affordable to do both sites in one long day. I also felt that our particular bus-company (!) guide was surprisingly talented. I will never forget her comment (many years ago) that "If you meet an older adult and they are of small stature, it's because they were malnourished during the Civil War." It is true that I would have liked to have more free time at El Escorial. But isn't much of the building "by tour only?"