Q:

Saying "sure" to reply to a stranger's "have a great day"?

Is it considered rude to say simply "sure" in response to some stranger's "great day"?

A:

It is customary to either return the kindness or offer some sort of appreciation.
"Thanks, you too," or simply "you too," is what I hear most often. It would also be acceptable to give a warm "thank you."
You will find that simply accepting the gesture—even though it's just routine politeness—sometimes throws off the other person. "Sure" or "okay" aren't typical responses. 
It's not rude, per se, but it's less than ideal.

