Formal:
* Bądź szczęśliwy/szczęśliwa (for male/female) - This is the most direct translation and is formal.
Informal:
* Bądź szczęśliwy/szczęśliwa (for male/female) - This is also used informally.
* Ciesz się! (Enjoy yourself!) - This is a more common way to say "Be happy" informally.
* Uśmiechaj się! (Smile!) - This is another way to say "Be happy" informally.
You can also use a phrase like:
* Niech Ci się dobrze wiedzie! (May everything go well for you!) - This is a general well-wishing phrase that conveys happiness.
The best way to say "Be Happy" in Polish depends on your audience and the context.