Formal:
* E ʻoluʻolu e leʻaleʻa i ka pāʻina. (Please enjoy the party.)
Informal:
* E ʻoluʻolu e leʻaleʻa. (Have fun/Enjoy yourself.)
* E ʻoluʻolu e hauʻoli. (Have fun/Be happy.)
More colloquial:
* E leʻaleʻa! (Have fun!)
* Hauʻoli! (Happy/Enjoy!)
The best option will depend on the context and your relationship with the person you're speaking to.