Lebewohl/Auf Wiedersehen...genieß deine Reise/deinen Urlaub auf Hawai und gute Rückfahrt/guten Rückflug nach Deutschland.

You can use either of the alternative where I put x/x

Hi, although the above answer is correct and all, I still would like to offer an alternative - a lighter, more colloquial way of saying this in German. It depends on who says this sentence to whom of course but among friends for example, I'd say this:

"Mach's gut, hab' viel Spaß in Hawaii und kommt gut zurück nach Hause."
(that's a very loose and literal translation but maybe it fits, who knows. If the person who is addressed with this sentence is not a native of Germany or doesn't permanently live there (i.e. if he wouldn't call Germany his home), you could replace "Hause" with "Deutschland".

Hhhmm, not sure after all if this helps a great deal but who knows?