How do you say "have nice vacations!" when ending a conversation with somebody who is going on vacations? (at work). "Schöner Urlaub"? I'm not sure, this sounds a bit cold / weird (it still has to be not too familiar as it's for colleagues, not necessarily friends).
It has to be in accusative as it's an object you wish someone to have.
That's okay for colleagues. For people you know better, particles add flavour:
(Einen) schönen Urlaub dann.
And of course, you can also use full sentences.
Dann (habt) mal einen schönen Urlaub.
Dann wünsch ich euch (noch) einen schönen Urlaub.
Since you are wishing the nice vacation onto him (I am not claiming that this is proper English), you should be using Akkusativ, just as in „Guten Tag!“, so it is
This seems to me to be ein similar to “have nice vacations”.