'‘Goodnight’' when leaving?

In the US, one might say ''goodnight'' when leaving a restaurant at the end of the evening. Does one say ''oyasumi nasai'' in the same situation, or is that only when one is going to bed? If it is not used in that way, does one simply say ''sayonara''?

When leaving a restaurant, you would generally say "Gochiso sama deshita" and also "Domo arigato gozaimasu". A sort of thankyou for the meal etc. etc.

Yes, you can say "oyasuminasai" when it's nearly bedtime when leaving a restaurant, bar or someone's home. Actually, I think it would be a lot more natural than "domo arigato gozaimasu", although "gochisosama" is most common.

Yeah, gochirosama is most of the time good. Though, at times it can be too formal. Hence, I may say "Are wa ooishi kata" ("That was delicious") or merely "ooshi kata".

In my opinion, it's perfectly normal for people to say "gochiSosamadeshita" even if you're 4 years old or 94 years old and even if all you had was a cup of tea.