I don't know what I should say when a guest say goodbye to me at the airport.
Can I say "Have a good journey" to him/her?
or
please suggest something to me. Thanks!

Question

Answer

Hi)))
Here’s how to say goodbye))
1)
Let`s keep in touch.
2)
Take care.
3)
Have a nice flight.
4)
Miss you already.

Answer

The whole question is, who is leaving?  If it's the guest, and I assume you are the one staying, therefore Ruta's suggestions all work.