Nativity scenes, which they call belenes in Spanish, are in most homes, and are not only the little things that you find in the US, although those do exist, they also include whole villages. They're really quite fantastic, and the pictures above are of the one we set up in the school.
Christmas trees aren't as popular, I think, although I see them around as well. The pictures above are of the tree we put up in the school and one in a train station, so they're around, but there aren't shops with tree ornaments springing up in the middle of town like the nativity scene shops that are there.
Lights! These are gorgeous. And they're on the main streets in the evening. It makes things feel magical and I love it.
I don't have pictures for this last one, but music is definitely existent too. If I was out more in the evenings I'd probably find more, but I've definitely found some groups singing Christmas carols, which they call villancicos in Spanish, and they're lovely.
So that's everything I've actually seen. But just to add one thing I do know about their celebrations, they traditionally receive gifts on January 6, from the Reyes Magos (the magi/three kings), not on Christmas. American influence, however, is making it so that something is given on Christmas, butI think the main gift giving is still on January 6.