Last minute Madrid itinerary advice requested …

There is also a botanical garden next to the Prado.
For snacks avoid Plaza Mayor, look on YouTube and find a channel by James blick called Spain Revealed, it will give you all you need to know about where to eat in Madrid

I'll add more later

When your tour of the Royal Palace is over, you can walk to a park north of the Palace’s Sabatini Garden and see an authentic Egyptian temple. It’s the Temple of Debod and it was dismantled in Egypt when it was threatened by flooding during the building of the Aswan dam. The temple was reassembled on a high location overlooking the city in Madrid’s Parque de la Montana.
The Temple of Debod has free admission so you can enter it and also see its interior.
Have fun in Spain!

As you come out of the Royal Palace, go next door and go inside the Almudena Cathedral. It's free, it's huge, it's beautiful. It might only add 30 minutes to your schedule because it is literally right next to the Royal Palace.

If you're unsure what to do after your refreshments, take a walk on Gran Via. I really enjoyed looking at those magnificent buildings.

Honestly, if you're not big museum people, have you thought about skipping the Prado? I know everyone will say you have to go, but you don't have to. If any of your friends ask you if you went to the Prado, just say, "Yeah, it was really crowded." If they ask what you liked, just say "Las Meninas."

For a snack near Plaza Mayor, Bar Campana do excellent Bocadillos de Calamares (squid butty or roll) Cheap, filling and delicious. Expect a queue of Spanish as well as foreigners. Take away is easier than trying to sit down.
Temple of Debod is indeed free but you have to book on the dedicated website....I know....because I didn't!
A stroll around mercado di San Miguel is fascinating even if you don't eat anything.
Hot chocolate at San gines for a second breakfast with churros (not far from Plaza Mayor)
We enjoyed just wandering around - but here are some more ideas
https://slowtravelitalyspain.blogspot.com/2025/02/what-to-see-and-do-free-in-madrid.html

There’s nothing wrong with snacks at the Plaza Mayor if you simply want to sit and enjoy the plaza. It’s only tourists who dine there, prices are elevated, and the food just ok. Roma will come by requesting money or trying to sell you something. Pickpockets are all over that area so keep your valuables in a safe place.

Just to add...

For tapas, or really Raciones, head to Calle de la Cruz, and a trio of places, Casa Toni, Casa del Abuelo, and La Oreja de Jaime. Between the three you have more choices than you can eat.

We had a very good paella one night at Arroceria D'Stapa, they did an excellent job. Hard to find good paella in Madrid, you really have to go to a specialty restaurant, probably run by Valecians.

For markets, Mercado San Miguel is fun, can try different things by pointing, prices are a premium, but not too bad. I would not get a meal there, just drinks and snacks. The problem is that it is usually so packed that you can not find a place to stand or sit, or even walk through. Mercado San Anton, not that far away, is much more relaxed and has a number of independent food stalls on the upper floor.

The Crystal Palace was undergoing renovations in March but El Retrio park is worth walking through.
Mercado San Miguel a food market is nice.
Faro de Moncloa observation deck offers a great view of Madrid and there is a nice park nearby.
Agree on the Cathedral and there it has a crypt as well.
If you are there on a Sunday consider the El Rastro market.

Awesome suggestions! Thanks very much!

I'll be curious to know what you wind up doing.

We really enjoyed the Royal Collections Museum next to the Palace. Lots of interesting stuff from carriages to clocks. The Sabatini Gardens were hard to get into because of the construction, but they are accessible behind the Cathedral. There are a series of ramps to take you down. It was very peaceful. We loved walking around, and eating in the San Miguel market near plaza Mayor. There is the Royal Botanical Garden near the Prado. We wanted to go, but the line was very long. But Retiro Park was wide open and a great place to walk.

If you're in the Plaza Mayor area and like mushrooms, you have to try Mesón del Champiñón www.mesondelchampinon.com
It was one our favorite places last year on our trip to Madrid. They cook mushroom caps on a flat top griddle and each mushroom has a little piece of chorizo in it. Delish!