Formal:
* Desfrute das suas férias! (This is the most common and formal way to say it.)
Informal:
* Boas férias! (This is a more casual way to say it, literally "Happy holidays!")
* Aproveite as suas férias! (This means "Make the most of your holidays!")
Other options:
* Tenha umas boas férias! (Have a good holiday!)
* Divirta-se nas suas férias! (Have fun on your holidays!)
You can choose the best option based on the person you are talking to and the situation.