Until December 23rd you can find original and traditional Christmas decorations at the Fira de Santa Llúcia.
Fira de Santa Llúcia is a huge Christmas market held in front of the Cathedral of Barcelona since 1786. It is made up of almost 200 stands which evidence how Christmas is celebrated in Barcelona. There you can find typical decorations like figures or eucalyptus for the nativity scene. The funniest ones are caganer and pixaner representing a man that poops and piss hidden in the bush.
Here, children do not receive presents from Santa Claus but from a friendly  trunk that poops presents if they hit them with sticks and sing a song. This dude is called Tió de Nadal and you can see a huge version of it at Fira de Santa Llúcia.
If you want to learn more about Christmas traditions in Catalonia, don’t miss 24 Diada dels tradicions i costums nadalencs a Catalunya (24th edition of Christmas traditions and customs in Catalonia). A complete immersion in our habits during these days. If you have children, make them enjoy with the Centenary Giants parade (11:00), puppet shows (12:15) and why not introducing Tió de Nadal to them?
If you still want more Christmas plans, it worths to get lost in the Gothic Quarter to end up in La Rambla. Its Christmas lighting, in honor to 17A terror attack victims and the model behaviour of neighbours and trader during these days are really emotional.