Qué tengas UN bien viaje!

Qué tengas UN bien viaje!
 

When do you use tengas instead of tiene?

"Never," would be my first response. The former is a second-person conjugation and the latter is a third-person conjugation. Either remove the 's' from 'tengas' or add an 's' to 'tiene' to keep the persons the same. ;)

I suppose you realize that 'tengas' is cast in the subjunctive mood and that 'tienes' is in the indicative mood.

The indicative mood is used for real events.

The mood is shifted to the subjunctive when you are wishing something, or if the event is unrealistic or dubious. A verb expressing emotion is another trigger, as is denial.
There are certain phrases that always trigger the use of the subjunctive mood, as well, like 'Ojalá (que)', 'para que', 'antes (de) que', 'con tal (de) que', 'a menos que', 'en caso de que' and 'sin que'.
Other phrases prompt the use of the subjunctive mood when the action hasn't happened yet or isn't completed, like 'después de que', 'hasta que', 'en cuanto', 'mientras que' and 'tan pronto como'.
And there are impersonal phrase that trigger the use of the subjunctive, like 'es bueno que', 'es importante que', 'es impossible que', 'es malo que', 'es mejor que', 'es necesario que' and 'puede ser que' (and several others).

In your example, someone is wishing something on another (that you have a good trip), so the subjunctive mood must be used.

There is no accent mark on the first word in your example. 'Que' is a conjunction, while 'Qué' is an interrogative. The difference is 'that' versus 'what'.

Don't forget that an inverted exclamation point should begin a phrase that ends in an exclamation point. ;)

I agree with Rusty. I'll only add it's "buen viaje", not "bien". Although it may be a typo, these are two very different words. ;)