How do you do? / Nice to meet you. / Spoken only when meeting someone for the first time, and never again.
Don't get "Nice to meet you"! confused with "Nice to see you again"!


How's it going? / How's it (day) going? / a general, daily greeting
How's the day treating you so far?


Have a good time! / should not be used unless someone has conveyed that they are on their way to do something special after you leave each other.


Rather: Take care / Take it easy / See you later / Catch you later /