Formal:
* Passez un bon week-end! (This is the most common and polite way to say it.)
* Bon week-end! (A shorter version of the above, still formal)
Informal:
* Bon week-end! (This can be used with friends and family.)
* Profitez bien de votre week-end! (Enjoy your weekend!)
* Amusez-vous bien ce week-end! (Have fun this weekend!)
You can also add a personal touch by saying:
* J'espère que vous allez bien vous amuser ce week-end. (I hope you have a lot of fun this weekend.)
* Bon week-end, et reposez-vous bien! (Have a good weekend and get some rest!)