For me, analyzing philosophical thought or a scientific theory will do.

Literature, like the other suggested, works too.

Avoid touchy subjects, like politics or classes geared towards a certain group of people (GLBT studies, gender studies, African American history, etc.) that may not apply to everyone.

Of course, I would need to know exactly what kind of speech you are giving to further help you out.

Maybe study college statistics, like advantages of online education vs. traditional education and disadvantages.

Psychology is a good subject too, and if you’re allowed visual aids, I would suggest optical illusions. Give the audience access to them tell them how to see them, and explain what they should see and stuff.

Don’t be afraid to email me if you need further assistance.