Which Attraction Tickets Ahead of Time for Madrid / Barcelona?

If you have time, We flew from BCN to Palma Mallorca for 1 night , really beautiful, and Ryanair return was cheap

You should also see National Art Museum and Poble Espanyol.
if there are no water restrictions see the magic fountain at night in front of the National Art Museum

Just to clarify - We are not really looking for new ideas on things to do. We have so many we probably won't get to them all. I didn't want to give a whole list to clutter things up. Patrick of 'Patrick Guide Barcelona' emphatically advises to get Sagrada Familia and Park Guell tickets on-line early even with the constraint of specific time slots. That's the kind of info I was looking for about major attractions. Without that I will pretty much get any tickets I can early. This might be too much detailed planning and lends to feeling on schedule instead of vacation. Trying to find that balance.

The Royal Palace in Madrid is on my list for advance tickets EC - all 10 sites listed are. Just wanted to know if there should be more.

We were just in Spain 2 weeks ago. We bought advanced tickets to the Royal Palace in Madrid, the Prado, La Sagrada Familia Parc Guell, the Picasso Museum , Casa Battlo and a flamenco show in Sevilla.
We had tickets for 11 am in Parc Guell and when we went there there was signage that all the tickets were sold out for the rest of the day. The only place we did not have advanced tickets was for the Palau Musica in Barcelona . We decided to go there at the last minute . There was no line and we walked right in.
I believe TemploDebod may be closed

Debod temple is free to enter. It is closed on Mondays. It is open from 10am - 8pm with the last entry at 7:30pm.

If you can’t get advance tickets to the Prado, consider going to it or the Reina Sofía during the free admission hours. The Prado’s free hours are, Monday to Saturday, from 6:00 pm to 8:00 pm, and Sundays and holidays, from 5:00 pm to 7:00 pm.

Reina Sofía free hours, Monday 7:00 - 9:00 p.m., Wednesday-Saturday, 7:00 - 9:00 p.m. Sunday 12:30 - 2:30 p.m.

The Royal Palace has free hour for EU citizens. How strict it is about the EU citizen rule is unknown. From Monday to Thursday from 17 to 19 (16 to 18 in winter).

You need to buy tickets in advance for La Sagrada Familia, Parc Guell, Casa Batllo (in general; I don't know about the "rooftop show"), Casa Mila/La Pedrera, Picasso Museum and Palau de la Musica Catalana (because the English-language tours sometimes sell out, and they don't run constantly).

I'm with you on hating to pin down my sightseeing ahead of time, but if you do not buy tickets for those places in Barcelona (with the possible exception of Palau de la Musica Catalana), you will be standing in a long ticket line (quite likely over an hour). When you at long last get up to the ticket counter, you may be told all the day's tickets have been sold or that you need to come back in 3 hours--which really messes up a tourist's day. It is unfortunate, but it's just the way it is.

If you can bear to do it, slot one of those problematic sites first thing each morning so you don't have to worry about being on time for them.

As far as I know you don't need to book the Montserrat trip ahead of time. You just go to the train station beneath Placa de Espanya and buy a package ticket there.

For Madrid I wouldn't expect issues at Temple de Debod.

I know nothing about the flamenco show.

I think you can save a bit of money by buying the Segovia train tickets early. Compare today's/tomorrow's fare on renfe.com to what you see for the first week of June. The difference is your approximate potential savings. I have not heard that those ticket sell out. (I know the trains to and from Toledo do sell out.)

You might be OK to wait for the Prado ticket, but I cannot guarantee that. I'm not sure it's possible for the museum to sell out--does it even sell timed tickets? But you could conceivably be in a line that would take some time to clear.

The Palacio Real may be somewhat riskier than the Prado. It seemed more popular when I was in Madrid in 2016. A ticket line wouldn't be surprising. My bet is that weekends and holidays are riskier than weekdays if you want to roll the dice.

Although it doesn't require pre-booking as far as I know, one of our other posters has warned that you won't get into the Naval Museum in Madrid without your passport.

We just left Barcelona on Saturday and are currently in Madrid. It looks like you know what you need to pre-book in Barcelona. Additionally, the Barcelona Cathedral was using timed entry tickets on Saturday and our entry was 30 min.later. We walked up to Palau Guell ( not to be confused with Park Guell) and that was a 60 min.entry wait. We had pre-booked the first English language tour for the Palau de la Música Catalana and there were only 15 people but the next tour was double that.

A couple in our traveling group went to Montserrat on Friday. They hadn’t bought tickets ahead and struggled as to where to buy at the station. The trains run hourly, so it helps to know the schedule. They missed the train by a minute then had to wait and crowds were heavier by the time they got to the Rack railway. You do need to reserve if you want to listen to the choir.

In looking at the website for the Royal Palace, it looks to be sold out Jun 19-20, so you may not want to wait on that. The Prado is still looking wide open.

Since acraven mentioned it, the Naval Museum was a pleasant surprise. We were walking by it last September and just entered. I don’t recall if we had to show passports. There was no line. The museum is free to visit, but it does ask for a nominal donation. It was very much worth it.

Appreciate everyone's feedback. I will follow my initial intuition and just get everything I can online before the trip. Much easier to do this on the big screen at home as opposed to a cell phone abroad. This won't remove all flexibility - at least two sites with fixed time entries can be combined in a day and not all tickets are fixed time.

This has the added benefit of 'skip the (ticket) line' but I wonder how easy it is to figure out which line is which. Different for different sites I guess. We'll have to figure it out when we get there.

In places where it could be an issue, there may well be a line-minder who will point you in the right direction. If there isn't, one of you can walk up to the door, show the ticket and ask.

The entry signs giving directions for La Sagrada Familia were easy to follow. They checked times and turned away those who had a different time.

Casa Batilo has signs for different timed entries and there was a line minder at the head of the line near the door.