First of all, almost any informative topic can become a persuasive topic by adding a call-to-action step. Secondly, try to avoid speech topics that deal with fixed beliefs like abortion, capital punishment, religion and so forth. People have spent many years deciding what they think about these topics and you are not likely to change their minds in a four to six minute speech. Don't set yourself up to fail.
School related Topics
Student finacial aid policies
Black sororities and fraternities
Health related topics
Eating less meat
Health care system
Birth control / Safe Sex
The importance of wearing well fitting shoes
Government related topics
Legalization of marijuana
The national debt
Legal drinking age
Nursing home regulation
Public office term limits
Why you should vacation at Walt Disney World
Persuading the audience to visit a local museum or zoo
Why you should watch...
Why we should write the Fox network to bring back Shannon Doherty on "Beverly Hills 90210"
Why you should read John Grisham, Stephen King, etc.
Equal rights / Sexism