How about tackling the issues of responsibility and freedom? It might not be as easy a topic as the medicinal uses, but I think if done properly it can be the strongest arguments for legalization.

What right does the government have to tell me what I should be doing with my body and my life, what substances I should be putting into my own body? Don't I have the freedom and the right to control the contents of my brain, to control the way I think and how I perceive the world? Shouldn't I be the one accountable for what happens to myself? Why does the law dictate that only certain states of consciousness are allowable and not others?