Formal:
* Счастливого пути! (S-chastlivogo puti!) - This is the most common and literal translation, meaning "Happy Journey!"
* Удачной дороги! (Udachnoy dorogi!) - This means "Good luck on the road!"
Informal:
* Хорошей дороги! (Horoshey dorogi!) - This means "Have a good road!"
* Счастливого пути! (S-chastlivogo puti!) - This is also used informally.
* Путешествуйте с удовольствием! (Puteshestvuyte s udavol'stviem!) - This translates to "Travel with pleasure!"
You can also use a phrase like:
* Берегите себя! (Beregite sebya!) - "Take care!"
* До скорой встречи! (Do skoroy vstrechi!) - "See you soon!"
The best choice will depend on your relationship with the person you're speaking to and the situation.