Formal:
* Passez une bonne vie. (This is the most literal translation and is appropriate for formal situations.)
* Je vous souhaite une vie heureuse. (This means "I wish you a happy life.")
Informal:
* Bonne vie! (This is a shorter and more informal way to say it.)
* Profite bien de la vie! (This means "Enjoy life!")
For someone you know well:
* Passe une bonne vie! (This is a more casual way to say it, and is appropriate for someone you know well.)
You can also choose to express your wishes for a specific aspect of their life, like:
* Je te souhaite beaucoup de bonheur. (I wish you much happiness.)
* J'espère que tu auras une belle famille. (I hope you have a beautiful family.)
The best way to say "Have a nice life" in French depends on the specific situation and your relationship with the person you are speaking to.