A few examples of real-life adjective declination from a recent trip to Ulm
Rauchfreier Bahnhof
(“Bahnhof” is masculine, nominative; there’s no article; therefore “rauchfrei” ends in “er.”)
Tee mit doppeltem Rum
(“Rum” is masculine, dative; there’s no article; therefore “doppelt” ends in “em.”)