"Lunch in a good restaurant."
If I understood correctly: It would be: Das ist ein gutes Restaurant. Mittagessen in einem guten Restaurant.
It changes because you are talking about the restaurant generally in one case, and talking about someone/something being IN it in the other case. Words like with/in/from make it change.
Danke, I can't tell the accusative case from nominative case. I understand accusative takes an action. In this case it is dative. I keep getting questions with case wrong!