Make your students form an argument as to why something is better than the alternative. Of course, you should first go over the structure of an essay, and maybe various phrases one could use in an argument. But to prompt them for further practice, here are twenty topics. They should choose which of a pair to support, and their arguments could be subjective, objective, or both.
Books vs. Movies
Night Owls vs. Early Birds
Hot vs. Cold Weather
Public vs. Private Transport
Cats vs. Dogs
Sweet vs. Salty Snacks
Folk vs. Pop Music
Calling vs. Texting
Still Art vs. Performing Art
City vs. Countryside
Pens vs. Pencils
Health vs. Happiness
Youth vs. Experience
Coffee vs. Tea
Superman vs. Batman
Skill vs. Hard Work
Mountains vs. the Beach
Dressing Up vs. Down
Fruits vs. Vegetables
Chill vs. Lively Venues
Note that some of these points are not mutually exclusive. But for the sake of writing an argument, students will have to choose one over the other.
Also note that these are good discussion points. Before or after the writing, perhaps you could hold a class discussion in which students takes sides and (civilly) argue their positions.