"Here's this app, here's what it does. How would you increase downloads?"
A quick brainstorm session shows you how they approach the situation, what tricks they have in their bag, and what methods they feel confident using (or if they're just BSing).
Interestingly enough, 50% of the time people answer "SEO," which is among the worst answers you could possibly give. The best answers often involve the person playing with the app and figuring out something you could actually build into the app itself.