Good Night
Rest well. (An informal and caring way to wish one person a good night's sleep.)
Sleep well. (Very similar to 'Que descanses,' a common and friendly way to say good night to one person.)
Happy night. (A warm and friendly way to say goodbye in the evening, even if not going to bed. Very common in Colombia and Venezuela.)
Until tomorrow. (Used specifically when you know you will see the person the next day.)
Good night, Mom. I'm going to sleep now.
Goodbye, friends. Rest well!
Thanks for the dinner. Have a good night.
It's late now. See you tomorrow, sleep well.
Upon entering the restaurant: 'Good evening, a table for two, please.'
'Buenas noches' is universally understood. It uniquely serves as both 'good evening' (a greeting when arriving somewhere at night) and 'good night' (a farewell when leaving or going to bed).
While 'Buenas noches' is used, 'Feliz noche' is an extremely common and friendly alternative used as a farewell in the evening, similar to 'have a good night'.
'Buenas noches' is the standard. The informal 'Que descanses' is very frequently used among friends and family when someone is heading to bed.
Usage is identical to Latin America. 'Buenas noches' is the standard for greeting and parting at night. The only difference is that when addressing a group informally, they might say 'Que descanséis'.
Saying 'Bueno noche' or 'Buena noches'. — The adjective 'bueno' must agree in gender (feminine) and number (plural) with the noun 'noches'. The correct and only standard form is 'Buenas noches'.
Thinking 'Buenas noches' is only a farewell. — Unlike the English 'good night,' which is only used when leaving, 'Buenas noches' is also a greeting. You use it to say 'hello' when you arrive somewhere in the evening (e.g., a restaurant or a party).
Using 'Buenas noches' too early. — This phrase is for when it is dark outside, generally after 7 or 8 PM. If the sun is still out, even in the early evening, it's more appropriate to use 'Buenas tardes' (good afternoon).