I responded with, “Thanks, you, too.”
The expression “Have a great day” is common and friendly. Time to change it.
There’s a bit of passivity in “Have a great day,” even though the verb is imperative. To switch to a more active thought, let’s use, “Make a great day.” Or, better, “Make the day great.” I think I prefer the latter. Change the verb to make, and use the definite article (the) rather than the indefinite a.
Now, go make the day great. Go make your day great. Go make someone else's day great.