Formal:
* Profitez bien du cadeau. (This is the most literal and formal translation)
* Je vous souhaite de profiter pleinement du cadeau. (This translates to "I wish you to fully enjoy the gift" and is more elaborate)
Informal:
* Profite bien du cadeau! (This is a more casual way to say it)
* J'espère que tu vas bien apprécier le cadeau! (This translates to "I hope you enjoy the gift!" and is friendly)
Other options:
* Prenez plaisir à utiliser le cadeau. (This means "Enjoy using the gift.")
* Bon usage du cadeau! (This translates to "Good use of the gift!")
You can choose the best phrase based on your relationship with the recipient and the occasion.