To me this sounds very unnatural. We rarely use "holiday" in the plural in this context, rather, we usually say along the lines of, "have a nice holiday," even if during that holiday you're travelling to more than one place. If you were to use it in the plural, we'd use it in the sense "in the summer holidays, I'm going to Spain with my family", however we wouldn't then reply "well I hope you enjoy your holidays", we would say "holiday". If you really wanted to say "holidays", it makes sense to say "enjoy THE holidays" as it's more general.

No. Do you mean 'holiday' as in the British-English word for 'vacation' or the American-English way of referring to Christmas time? If you mean 'holidays' as in multiple vacations, it's grammatically correct, but I can't imagine a situation when someone would say that. It would be "enjoy your holidays." If you mean "holidays" as in "the holiday period", then I believe Americans just say "Happy holidays" or "Have a nice Christmas".

You can say that, but I rarely hear it said that way using "nice". it is more common to say "Enjoy your holidays" or "I hope you have good holidays".

"Have a nice holiday"