Saying さあ means you are expecting the listener to do or start something in response, like "hey" or "come on". So "さあ、おやすみなさい" would sound like you're urging someone, like "Hey, time to go to bed!" If you just want to add the nuance of "okay" or "now", それじゃ/じゃあ/それでは/では is the correct choice.
それじゃ/じゃあ/それでは/では is used to indicate it's time to go (back) to an important/original topic after desultory chatting, silence, greeting, etc. For example, saying "それじゃ、お茶をどうぞ" is fine when:
- you've been chatting with your guest for a while with a teapot in your hand
- the tea itself is the main purpose of the gathering
But if you said it in an inappropriate situation, it would sound puzzling, and your guest might wonder if the tea has some special meaning. (それじゃ/etc also means "in which case", but that's not relevant now.)