This is a social propriety question and not a language question. But in that spirit, here's my answer:
If you're a non-Christian and don't celebrate Christmas, or if the person you're speaking to does not celebrate Christmas, then just say "hello"! Why in the world do people look for a way to bring up a specific religious holiday without mentioning the holiday?
As a Christian, I find it highly offensive when people say, "Happy Holiday Season" or the like. What holiday is it that you are referring to? So why can't you say "Merry Christmas"? Do you hate Christians so much that you can't even bring yourself to SAY THE NAME of our holiday?
I have no problem saying "Happy Hannukah" to Jewish friends. I once sent a Chinese friend an e-card for Tet. I'd be happy to give Ramadan greetings to a Moslem friend if I knew the appropriate greeting. ("Happy Ramadan" seems inappropriate for what is a pretty somber holiday, but I don't know.)
I often hear people say that "Happy Holidays" is "more inclusive". This is simply absurd. The whole point of saying "Happy Holidays" is to refuse to acknowledge Christmas. That is, the whole point is to EXCLUDE a specific group.
Suppose that a store put out an ad on Martin Luther King Day that said "Mid-January Holiday Sale" and which pictured only white people. When asked, they explained that they didn't want to refer to it as Martin Luther King Day or to include pictures of black people because they were afraid that that might offend Nazis and Klansmen. Would you say that that store was being "inclusive", and praise them for their tolerance in avoiding such offense? Or would you say that they were pandering to racists and bigots?
Likewise, I think most non-Christians understand that Christians celebrate this holiday called "Christmas", and while they do not celebrate it themselves, they don't begrudge others having their own holidays. To say that someone else should not be "allowed" to have a holiday that you do not celebrate, or to go out of your way to pretend this holiday doesn't exist, is simply rude and intolerant.
If you want to wish someone a merry Christmas, say "Merry Christmas". If you don't want to acknowledge Christmas, than just say "Hi, Bob!" or "Wow, cold weather we're having, isn't it?" But bringing up someone else's holiday and then making a pointed effort to not acknowledge it is just insulting.