Formal:
* Penwythnos da i chi! (Pen-wyth-nos da ee kee) - This is the most formal way to say "Have a nice weekend" and translates literally to "Good weekend to you!"
Informal:
* Caer dy benwythnos! (Kair the pen-wyth-nos) - This means "Enjoy your weekend!" and is a more casual way to wish someone well.
* Penwythnos braf i chi! (Pen-wyth-nos brahf ee kee) - This translates to "Nice weekend to you!" and is a good option for a friendly, but not overly familiar, greeting.
Adding "Have a nice weekend" to a longer sentence:
* You can also use the phrase "gobeithio y bydd gennych chi benwythnos braf" (go-bay-thee-oh ee booth gen-nukh kee pen-wyth-nos brahf) which means "I hope you have a nice weekend."
Ultimately, the best way to say "Have a nice weekend" in Welsh depends on your relationship with the person you are talking to and the context of the conversation.