Formal:
* Cael amser da: This is the most literal translation and the most formal option.
* Mwynhewch eich amser: This means "enjoy your time".
Informal:
* Cawsoch chi amser da: This is a more casual way of saying "have a good time".
* Mwynhewch! This means "Enjoy!" and is a very informal way of saying "have a nice time".
More specific:
* Mwynhewch y parti! (Enjoy the party!)
* Mwynhewch y digwyddiad! (Enjoy the event!)
* Mwynhewch eich gwyliau! (Enjoy your holiday!)
The best option for you will depend on the situation and your relationship with the person you are talking to.