Christmas and Chanukah
Cross-religion education is a good thing, and I think most people would welcome serious inquiries from people of different faiths if they want to know more (or even attend services or other rituals). I've certainly participated on both sides of that and seen no problems. But I think we should remember that our religions are separate; they have different emphases and that's ok. We don't have to agree, and we don't have to try to build a lowest common denominator.
To my Christian friends, I wish you the best in your season of holiness now under way, as I know you wish similar things for me during the high holy days and at other times. I'm not offended by your observance of your religion; you don't need to water it down. Besides, the dreidel song is really insipid; please don't feel obligated on my account.
(Mind, I would have a different reaction to celebrations in a setting that's supposed to be neutral, like a public school or a place of (secular) employment. But that's not what I'm talking about here.)
[1] Some do not see a problem with things like singing Messiah. We all draw the "worship" line in different places.
Re: Holiday greetings
I generally assume that "merry Christmas" has taken on the same role as "have a nice day" (now escalated to "have a great day" in many places) -- a rote greeting that usually does not pass through the deliberation phase of the brain on the way to the mouth. It's December, and that's something a lot of people just say in December. I used to respond in the ways you talked about, but I've come to the conclusion that the cashier at the grocery store doesn't really care, and the socially-correct response is along the lines of "same to you". For someone I'm closer to, where it would be appropriate to drop the "actually I'm not" hint, I'll say something different.
I usually side-step the gift thing by just handing the person the package and saying "I'd like you to have this". :-) Or, if it's my parents, it's pretty clear that it's a Christmas celebration so shrug, but if they can keep the family get-together Jesus-free on my account, I can at least admire their tree and not be unhappy if there are reindeer on the gift-wrap.
Re: Holiday greetings
Just before Thanksgiving, I took to saying ,em>Have a nice holiday to the patrons. I figure if most people are off work, it's a holiday. I plan to do the same right before Christmas.
Re: Holiday greetings
You know why Christians promote Christmas so heavily? Misery loves company, that's why. (Only half joking.)