I am atheist, and DH is buddhist, and we have only celebrated christmas since dd (2.8) came along.
We do 'traditional' type things- we go together as a family to choose a tree, decorate it together, have christmassy programmes on the radio, stockings etc. but don't go overboard on gifts- its about spending time together as a family.
Christmas day, we have always had MIL over for lunch (14 yrs now), and still do- cooking and eating together is important in our house- everyone pitches in.
I want to teach DD about the origins of christmas, and so this year, we will hopefully have a nativity scene.
We weren't going to tell her about father christmas, but she goes to nursery full-time, and around early december last year she stunned me with "Mans going to come and bring presents, mama" so that cat was out of the bag. To be honest, I downplayed it as much as possible- her presents were from us or the family, not FC or SC.