I'm teaching subject-verb agreement and I want my students to apply the rules in writing an essay. But my problem is, what essay topic can I give the students that will make them use subject-verb agreement? Thanks a lot.

I don't know how you would make students use a particular rule in an essay unless your writing prompt called for students to
1) create an example (1-2 sentences long) using a singular subject
2) change the subject to plural and rewrite the example changing the verbs to agree.
3) discuss what they did and why.

One of the examples on the page noted below has something along that line.