We're travelling to Barcelona, Valencia and Palma de Mallorca in October. Other than the Sagrada Familia, what sights should we consider booking tickets in advance? We plan to go to the zoo and aquarium with our 2 year old grandson likely wherever there is one.
I think there's a large aquarium in Valencia, but I know nothing about the ticket situation there; I'm aware that some aquariums are very popular. I've also never been to Mallorca.
In Barcelona these are sights for which you need to buy tickets in advance--not necessarily because they always sell out so far ahead (though I'm sure this is more of an issue now than it was for me in 2016), but because the ticket lines are horribly long. For an October trip, I don't think you need to buy tickets anytime soon. It's best not to commit sightseeing money unnecessarily early, because sometimes trips get postponed or cancelled.
La Sagrada Familia
Parc Guell
Casa Mila (La Pedrera)
Casa Batllo
Picasso Museum
Palau de la Musica Catalana (English tours can sell out)
You should expect very serious crowding in the first five.
We went in November and just booked the night before or usually on the day of the visit in order to keep our schedule flexible and just do attractions when we felt like visiting. But November was definitely low season and less crowded. I suggest that you just check online the month(s) and week(s) before to see how fast tickets are selling during your schedule and book accordingly.