Making up maps on the fly is an important skill for a GM. Because you know that even the most casually described location is going to get searched exhaustively and the players will want to know the details of every building and room and tree and rock to make sure they don't miss any secret doors or hidden loot.
A good diversionary tactic to use to give yourself more time to come up with this stuff is to suddenly demand a saving throw. If you make the reasons for it mysterious and not fully revealed, that should keep the players occupied with other things for a good few minutes, and may even make them want to go somewhere else - like maybe the area you have prepared in advance.
C-3PO: I am C-3PO, human-cyborg relations. I hear you're having trouble with your droids.
Lando: No, no problem, why?
C-3PO: Hmmm. I download a map of Cloud City.
GM: <sigh> I'm actually going to have map this, aren't I?
Han: We were going to go everywhere anyway.
GM: Yes, but it was originally going to be a one-room spaceship in orbit.
C-3PO: You're welcome!
GM: You've been outside a few minutes. Save versus poison at -4.
Chewbacca: What? Why is everyone walking around without masks on?
Lando: Why do you think we live here?
Chewbacca: I don't understand!
GM: Everyone who failed feels tipsy from the abundant alcohol in the atmosphere.
Chewbacca: I understand everything and I suggest we never leave.