How do you say goodnight in Spanish to a girl?

Buenas Noches.

Or maybe try some of these:

Hasta la vista. (Until I see you)

Buenas noches mi amiga bonita. (Good night my pretty friend)

Buenas noches señorita. (Good night miss)

Buenas noches señora. (Good night madam)

Hasta mañana. (Until tomorrow)

...or some combination of the above.