I have a strong suspicion that Barcelona will not be without tourists! There are a bunch of sights in that city for which we normally suggest purchasing tickets in advance so you don't risk a very long wait in line. I don't know how much, if any, better that situation might be in the lead-up to Christmas; Barcelona seems to be busy year-round. At the very least, I'd arrive in the city with a list of the top places I wanted to see and consult the tourist office about which might need to be pre-booked. These are the toughies:
Picasso Museum (unbelievably mobbed even once you get inside)
La Sagrada Familia (decide whether you want to ascend a tower)
Casa Mila/La Pedrera
Casa Batllo
Parc Guell
Palau de la Musica Catalana (prebooked tour required, English available)
Camp Nou (soccer thing; I suspect that it sells out year round)
Despite the length of that list, Barcelona has many great sights where you can just walk in.