Just for info for the "more adventurous" when visiting Barcelona:
Due to the sheer number of visitors to this city happening all year long, many on tourism but also for a myriad of other reasons (business, special events, etc), several of the most popular tourist sites have introduced some years ago -same than in other popular European destinations- the "time-band" booking systems.
For the most popular sites, pre-booking online is almost mandatory, it's very rare to find tickets in the manned booths at the site for immediate visitation. Visiting hours are split into "slots" and there's a maximum capacity of visitors every hour, not only for safety reasons but also to ensure a comfortable visit for the customers.
When pre-booking online, one chooses one slot, which indicates the time one MUST be at the entrance, ready to go in. The window is (normally) 15', miss it and you need to purchase another ticket to visit. Once inside there's no limit to the time you can be admiring the site, so you can spend an hour, two... whatever. Having said that, those sites (ie Casa Batlló) in which you don't roam freely but in a sort of pre-established itinerary (much like in any Ikea, if you know what I mean!), you can't really spend that much extra time because you're being "pushed", so to speak, by the group behind you. This is not to say you can't stay in a room longer if you want to, but in practice, you'll feel you're being pressed to move on after a reasonable period of time.
So, if you think "bah! I'll just buy a ticket on the spot -or online, on the phone, when I get at the site" know that you'll most likely find that your next available "slot" is hours away, or even the next day. That's how busy most of the most popular sites are.
Having said all that, the expert management of the flow of visitors at these sites makes it reasonably comfortable to visit.
As mentioned, some of the sites that are advisable to pre-book are: Sagrada Família, Casa Batlló, Park Güell, La Pedrera, Palau de la Música Catalana, Torre Bellesguard, Casa Museu Gaudí.... and a few more.
Enjoy!
PS: Disclaimer!... sometimes, at certain days, when planets are aligned in the sky and other heavenly happy coincidences occur, one can find tickets at the manned booths for a time-slot reasonably close to the time you're there.... even immediately. But again, you probably have more chances to win the Euromillions or the PowerBall, LOL!