Sometimes I like to say "I quite agree" when someone sneezes.
I haven't given up the habit either. If it makes someone else feel a little better when you say it, what's the harm? No one believes that a blessing is being put upon them. At least, I hope no one really believes that... I don't get offended when someone says "bless you" to me... but I'm a stickler for manners, so I find it nice that they said anything at all. People can be so rude! Haha! Other than saying something in another language, I can't think of anything else to say. I do like Jewelz "Do you need a tissue?"
Maybe a general question about their health would be appropriate?
But what would you say to a total stranger?