What museums/sites do we need rez prior to leaving US?

Sagrada Familia Barcelona

The Alhambra in Granada, for sure.

I always reserve every single thing that allows it! :)

I'm with Kim. I don't like waiting in line. I can only speak to Barcelona and it depends on which sites are on your list. But we were there in April and were very glad for our reservations at Sagrada Familia, and the Gaudi sites. Didn't do the art museums but saw some long lines as we walked by.

Gaudi sites needing reservations are La Sagrada Familia, Parc Guell, Casa Mila (aka La Pedrera), and Casa Batllo. Palau Guell isn't as heavily visited but sometimes has modest lines, apparently caused by capacity limits.

The Picasso Museum is an overcrowded nightmare even with a ticket purchased in advance; don't even think of going without a ticket. And don't go at all if you are only casually interested.

The Palau de la Musica Catalana can be seen only on a tour. You might have to wait for a later tour if you don't book in advance.

Despite this list, there are many wonderful sights in Barcelona that do not normally require advance purchase of tickets. I would not want to schedule back-to-back timed sights (and all of the above are timed, though some offer more expensive wild-card tickets), because how on earth can you predict how much time you'll need at each sight (except the tour of the Palau de la Musica Catalana)?

It'll be far easier if you mention which sites do you intend to visit while in Barcelona as there are a number of them in which it's advised to prebook.

In Barcelona ...
•Sagrada Familia
•Palau de la Musica Catalana
•Park Guell
•Picasso Museum

In Granada ...
•Alhambra

In Sevilla ...
•Real Alcazar (maybe)

Thanks everyone. You've been helpful!