Yes.It's okay to say I hope you will have a good time

I´m learning too. Sasha. I think that it is correct: Enjoy your holiday. I hope that you have a good time. "will have" it is future, and you are affirming this.

In your comment you say 'I think we can't use "will" in the sentence. But I can't explain why'. You might be thinking of a different context, as in adverbial clauses introduced by "if", "when", "until", and the like. For example, we say "When you arrive in Paris, give us a call", not "When you will arrive ...". "I hope" however creates a noun clause, and it can have "will" to express a future event. Here's a passage from a song ("Desperadoes Under the Eaves") that illustrates this point. - And if California slides into the ocean, ("if" adverbial clause - no "will") - like the mystics and statistics say it will, (noun clause - "will") - I predict this motel will be standing until I pay my bill ("will" in the noun clause after"predict", no "will" after "until I")

You're welcome, Sasha.

Thank you, Jerry!!