The United Statesians are well known for their one-size-fits-all: "Have a nice day". Usually at the end of a retail-related conversation or transaction.
Over in France (a country that seems to pride itself on unnecessary complexity) the expression is the equally simple: "Bonne journée".
Unless it's the evening, in which case: "Bonne soirée".
Unless it's Friday evening, when you say: "Bon week-end".
Though on Sunday morning: "Bon dimanche" is the way to go.
And just before any meal: "Bon appétit".
Whatever have-a-nice expression should always be followed by: "Au revoir" (or "à bientôt" or "à tout à l'heure", depending), with each person playing the game, thusly:
PERSON A: "Bonne journée."
PERSON B: "Bonne journée."
PERSON A: "Au revoir."
PERSON B: "Au revoir."
Needful to say, Person B has the opportunity to say (instead of "Bonne journée"): "également".