We are traveling to those 2 cities in September. Are there any sights that need advance purchase before we go? I have been told we might need to for Sagrada Familia.
Well, the list of sights in Barcelona is quite long, so if you were to be more precise I can tell you which ones you ought to pre-book.
Sagrada Familia, Park Guell, Picasso Museum, La Pedrera, Palace of Catalan Music in Barcelona
Royal Palace, Prado Museum, Thyssen-Bornemisza Museum in Madrid
In Barcelona the following really are musts to prebook: Sagrada Familia, Park Guell, Picasso Museum, Palau de la Musica Catalana. In Madrid I think walk up is fine everywhere if you are savvy with your timings.
@sramey5... in Barcelona you hit five of the most visited, so try to pre-book all of them.
If there's a special exhibit at the Thyssen and you want to see it, it's a good idea to book in advance. There are often long lines to buy tickets at the Prado so buying in advance could save you that waiting time.