reservations to Sagrada Familia, Prado, other key sites in Madrid, Barcelona

It's risky to answer a question like that since tourism is up this year and demand can change from week to week. When is your trip?

In my experience (Madrid in May 2016, Barcelona in August 2016 and May 2019) the Royal Palace required less pre-planning than a bunch of sights in Barcelona, but the situation could be different for you. In any case, at the times of my visits tickets were available 1 or 2 days out. However, I had a generous amount of time in Barcelona, so I wasn't struggling to fit 3 days' worth of pre-booked, time-specific sightseeing into just 3 days.

Like you, I am really reluctant to book non-refundable things before I arrive in the city in question. If you're going to take that risk in Barcelona, I think you need to monitor all the ticketing websites daily for the last week or two, to be sure things aren't tightening up dangerously. The early time slots seem to sell out first, and making a short trip to Barcelona without getting any early tickets is more challenging. That first sight of the day is the only time you don't have to worry about how much time you'll spend at your previous stop and how long it will take you to get from Sight 1 to Sight 2. It's quite a challenge to figure out appropriate spacing when you have back-to-back timed tickets.

All these sights in Barcelona are potential problems:

La Sagrada Familia
Parc Guell
Casa Mila/La Pedrera
Casa Batllo
Picasso Museum
Palau de la Musica Catalana (English tours can sell out)

You don’t mention when you’re traveling, but for most of the year (and I only lack data for summer), if you’re flexible, you don’t need to worry much about the majors in Madrid. Weekends will definitely have less availability as they approach.

As acraven said, it partly depends on when your trip is. For at least the Prado and Reina Sofía museums, as a last resort, you could always go during the free admission hours if they get sold out. I’d try and wait until a couple days before visiting, if possible.

Thank you acraven, scudder, and jaimmelsabio, for your thoughtful replies. I just edited my post to include dates we'll be in Madrid and Barcelona, this March 15-23. coming up very soon. Had meant to include that originally, but goofed. Thanks again to all

Oh, that soon you're certainly fine for Madrid (comment regarding less availability on weekend stands.)

Thanks again Scudder