In our neck of the woods, it is no longer "Have a nice day."
It's "Have a good one."
I always wonder, but have never asked, "Have a good what?" While I presume "good one" is simply a substitution for "nice day", who can know for sure? Afterall, it could mean have a good laugh, a good cry, a good meal, a good ride home with your groceries, a good time mowing your lawn, a good homebrew (which goes without saying)... who really knows?
And I'm always reluctant to say 'thank you' because I have no idea exactly what "good one" it is this unknown 19-year old check-out associate presumes applies to me.