"Have a nice day." isn't a greeting: it's used for farewells. Here are some alternatives: "Have a(n) nice/good/great/wonderful/excellent/fantastic/brilliant/exciting/fine/successful day." Not all of these mean the same and some are more natural than others. Place any adjective you want where needed, these are just suggestions. "Have a good one." I don't think this is common outside Australia. Can someone confirm this? "I wish you a ____ day." This is more formal. "Ejnjoy." or "Have a good time." Only used when the other person is doing some kind of activity. This isn't constrained by time, so you could use it if the other person is about to see a movie or go on a six month exchange. "Enjoy your day." Same as the original. Other ways you can say goodbye: "Goodbye." or "Bye." They need a mention. "So long." or "See you later/next time." Both mean the same thing, except the latter is more casual. "Good night." Only used at night ie. late evening or early morning. "See you soon/shortly." "Farewell." Very formal and not common nowadays. "LIve long and prosper." The only other thing I can think of. This is only used jokingly.

Maintain good communication with friends is the best greetings!

others: Have a wonderful day. Have a fantastic day. Have a good day./Have a good one. (more casual) Vaya con Dios (Spanish for "Go with God") Have a Blessed Day. Peace be with you. (or casually just "Peace" or "Peace out.") God speed/Godspeed Farewell friends. (or an archaic way, but that might be fun for a "special" occasion: "Fare thee well") Until Next time...

yes...

Are you asking particularly about when saying bye to your friends?