Never sent or received a Christmas card
Most people do this as it's a nice way of reminding people that you're thinking of them. In a lot of cases, as everyone knows, a Christmas card is the only point of contact between acquaintances once a year. As Gordon has pointed out, they were/are a Victorian invention, as with so much else about Christmas - 1843, to be precise, an idea of Henry Cole (who along with Rowland Hill had been instrumental in creating the modern postal service as we know it a few years earlier) designed to get people using the system.
or been to a carol service
Never done that.
or nativity
Or that.
joined in with carols
Or that.
or just enjoyed listening to them
Often done that, as I love music. One of my heroes, Ralph Vaughan Williams (an atheist) collected and even wrote many, for the self-same reason. Listening to a Christmas carol no more makes one a Christian than listening to traditional Indian music (something else I do a lot) makes me a Hindu. It's just music.
given or received Christmas presents
Of course - it's gift-sharing season.
said "Happy Christmas" to someone, etc, etc.
That's an expression of good wishes along the same lines as wishing someone a happy birthday on their birthday, isn't it?
Any atheist who denies doing any, or all those things, is an arrant liar.
I was expecting a few examples of something specifically Christian about Christmas. Was that supposed to be it?