When people say, “Have a nice day,” it always strikes me as an invitation to be passive, to “have” a nice day, rather than to create one. It makes one the recipient, rather than the creator.
So I’d like to suggest using “Enjoy the day” instead. To me, this means to infuse the day with joy. To contribute to the joy of the day. It puts you in the driver’s seat, and gives you the opportunity to reflect on how many ways you can do that. A smile. A simple gesture like letting someone get in front of you on a jammed freeway. A call to inquire how a sick friend is doing. So many ways, all taking but a moment. And the more you bring joy, the more it reflects its light right back at you. So – enjoy your day, and everyone else’s!