Well, it depends on the individual. I celebrate secular Christmas, with Santa, reindeer, presents, a tree, stockings and a good dinner. (I know many of these traditions have religious origins, but are so far removed from their original context as to reasonably be considered secular)
Some people celebrate with manger scenes (I know there's a name for that, but I can't remember), church services, and my family had this wierd thing where we wrote a letter for Santa to take to Jesus.