Just say merry xmas...
I am not going to say thank you or wish anyone Merry Xmas if it is not what I celebrate...
If someone wishes me Merry Xmas, I usually say nothing...if they are persistent, I wish them Happy Hanukkah just to be contrary. Not worth the effort or time since it means nothing to me...
Happy Holidays - but even that pisses off the Jehovah Witnesses, so go with what you want I say.
EDIT - I would also suggest "Go Festivus Yourself!" is another appropriate response!
If happy holidays makes people mad, I'm just gonna start saying "happy shut the fuck up".
@TheGreatShadow You could also say, "Go Festivus Yourself!!!!"