Formal:
* Сладких снов! (Sladkikh snov!) - This is the most common and straightforward way to say it. It literally translates to "sweet dreams!" and is appropriate for most situations.
Informal:
* Хороших снов! (Khoroshikh snov!) - This translates to "good dreams!" and is a bit more casual.
* Спи спокойно! (Spi spokojno!) - This means "sleep peacefully!" and is a very sweet and comforting way to say goodnight.
* Приятных снов! (Priyatnykh snov!) - This translates to "pleasant dreams!" and is a more poetic and charming way to say it.
More affectionate:
* Сладких тебе снов! (Sladkikh tebye snov!) - This adds a personal touch, directly addressing the person.
* Спи хорошо! (Spi khorosho!) - This means "sleep well!" and is very warm and affectionate.
You can choose the phrase that best suits the situation and your relationship with the person you're saying goodbye to.