If I was in another city and saw them, I think I'd ask a few questions that don't appear too "JW savy". Like, asking a question on 1914, 1919 or 607 would immediately trigger their "apostate alarm". I'd like to ask some questions like:
When Jesus was there, why didn't he write any books? Did he asks people to build him churches?
I heard that you guys shun people, a lot of people. Is this true? Even family and very close friends? Why? Did Jesus ever shun people? How could he be tempted by Satan if he did not shun him? Why did he go in the desert in the first place knowing he could be tempted? What was the point of that? Wasn't that willfully putting his beliefs to the test?
If someone you know tells you that he's planing to kill someone tomorrow. Would you try to stop him? Would you inform the police? Why? How would you feel if you did nothing? Then why would God be able to stand by idle and watch so many bad things happen? I went to visit some concentration camps in Germany, I was amazed by how long it took the Germans to get to that final solution. They changed laws, organized trains, ghettos, build railroad, camps, etc. It took many years and God, who sees everything, knew exactly what was going to happen. Why? What did that change in the greater scheme of things? What is his excuse for doing nothing?
I'd try to get more random questions that are general, but easily puts questions in their minds.