I really don’t understand what the big deal is. Stop being so worried about offending someone and just wish them a happy or merry whatever, you’d be surprise the pleasant reaction you get in return.
I know personally myself I’ve been wished a Happy Hanukkah and even though it took me by surprise I had a lovely big smile on my face as I said it in return. Guess what it didn’t kill me and whatever higher power may be up there watching down didn’t strike me dead on the spot for being a decent human-being and wishing the nice old lady a Happy Hanukkah back. In fact her eyes kind of lit up that I didn’t get offended and actually said it back. A holiday is a holiday and in the end they are pretty much all about family, and last time I checked almost all the religions out there want us to be kind to one another so shove your beliefs a side for two whole minutes and if someone wishes you some sort of “happy holiday” of their own belief return it. After all if you said “Merry Christmas” to someone you would want them to say it back.