Barcelona is not a tourist resort so there are no high and low seasons. The city has visitors all year round therefore it's advisable to always pre-book despite the fact sometimes, some very specific days at certain hours, it could be OK not too.
Also note that the city is destination to a lot of foreign school escapades and also senior citizens trips from other European countries -both normally traveling outside summer months- therefore you never know when you're going to have a line of students/pensioners visiting your next attraction. Same goes for leisure activities on congresses and conferences, which we also host many. Just as an example, last week we had 101,000 visitors attending the Mobile World Congress all at once in the city... so some key venues in the afternoon/evening were packed to the rim. And of course, it's the first port in the Mediterranean, so cruise-makers are invading the city all the time too, and cruisers are mooring also all year round sometimes coinciding up to nine of those mega-monsters in the same day. And all of this without counting internal tourism -from other parts of Catalonia- and the Barcelonians themselves... our schools and pensioners also visit these sites of course.
So yes, at any of the most popular sites you might find yourself in a moment with 'fewer' visitors but that's only a mirage, LOL!
