I say Merry Christmas or have a Happy Christmas
I also say this to people I know who are of other faiths and I give them Christmas cards.
No one has ever taken offence and I think for the most part they are glad that they have not been forgotten in among all the festivities.
Most people from other faiths do enjoy the time off and time to be with their families even if they are not Christians and so it is still good to wish them a Merry Christmas.