Formal:
* Maak een goede reis! (Literally: "Make a good journey!")
* Veel geluk met uw reis! (Literally: "Good luck with your journey!")
* Reis veilig! (Literally: "Travel safely!")
Informal:
* Goeie reis! (Literally: "Good journey!")
* Veel plezier! (Literally: "Have fun!")
* Tot ziens! (Literally: "See you later!")
More specific:
* Rijd voorzichtig! (Literally: "Drive carefully!") - For someone traveling by car
* Vlieg veilig! (Literally: "Fly safely!") - For someone traveling by plane
* Veel plezier op vakantie! (Literally: "Have fun on vacation!") - For someone going on vacation
You can choose the phrase that best suits the situation and the person you are talking to.