More formal:
* A hui hou: This literally means "until we meet again," and is a common farewell.
* A hui hou kākou: This means "until we meet again, all of us," and is more inclusive.
More informal:
* E ʻike hou kākou: This translates to "we'll see each other again."
* Ke kali nei au e ʻike hou iā ʻoe: This means "I'm looking forward to seeing you again."
* E hoʻomanaʻo mai iāʻu: This means "Remember me."
The best choice will depend on the context and your relationship with the person you're saying goodbye to.