Prado Museum

It's only free in the evening, like 6-8 pm weekdays, 5-7 weekends .

When it is free, the lines seem to grow a bit longer than usual so go early. Even when it is free, you must line up to get your free ticket. They don't just throw the doors open.

I've only done coffee, but I'm pretty sure salads and sandwich type things were offered.

Good to know, thanks

If you google "best time to visit Prado" you will get plenty of suggestions -for example:
https://spanishsabores.com/2017/08/31/tips-for-visiting-the-prado-museum-in-madrid/

You can buy timed tickets ahead of time. There are free entry hours in late afternoons most days (M-Sat, 6-8 p and Sun/holidays 5-7p). Those are going to be busy times along with first thing in the morning. Details on entrance times, link to buy tickets, and costs are here: https://www.museodelprado.es/en/visit-the-museum That page also has several suggested itineraries to help you see the greatest hits (in 1/2/3 hours).

I think the Reina Sophia also has free evening hours.

Its website will have your answers: https://www.museodelprado.es/en/visit-the-museum
As already stated, the Reina Sofia museum, a few blocks away, also has free evening and Sunday hours.
There are a number of restaurants in the area. If you go to google maps, it will show some in the area of the Prado.

There is a restaurant, but somewhat limited and very expensive. The 7E dry cheese baguette sandwich is 2E elsewhere. There is a Mcdonalds a long block away, but a cafeteria type place across with a menu del dia and a generous wine jug. Numerous other eateries.
I spent 7 hours in Prado, but did the Sunday free day at Sophia Reina for 2 hours as that is all I was interested in. I don't rush museums. The Thyssen was an unexpected surprise. There was a Toulose Latrec exhibit at the time....very interesting, but could not doddle. It had timed entries.

The Prado is huge. Use their website and figure out what you want to see in advance, whether it's specific artists, periods, or schools. How much time you spend depends on your interests and stamina. I spent about 6 hours in the galleries on my first visit, about the same on my second.

We just went to the Prado today during their free hours from 6-8. We were in line a few minutes before 5 and behind a dozen people. Many many more lined up after us. I believe they all got in, but keep in mind everyone goes through security so I'm sure that some had no more than an hour there.
People buy tickets if they don't want to spend the time standing in line or they want more than a two hour visit.
We went to the Thyssen Museum today as well as Mondays are free from 12-4. Well worth a visit.
We walked right in at 1 pm. No line.

Unless you like the crowds or want to be rushed, buy a ticket. As for the cafe, while it has museum price food, my tortilla and cafe con leche were surprisingly good and it's a decent space. If you are 65+, you can buy the senior discounted tickets that will save you a bit of money.