You actually bring up the biggest obstacle to my tentative idea for this Sunday, of holding up a sign that points to a time/place for a casual conversation with strangers. I thought this would be a good way to get very lonely passers-by out of their comfort zone and into a situation where they have a chance to make friends and bond, but the absolute diversity of interests is the main show stopper. My first thought was to essentially avoid sensitive topics on the poster, such as religion and politics, but it still leaves the huge diversity of potential common interests open. So I started doing some research on the most common hobbies that people have in cities and that can be talked about casually, in hopes of finding like 5 ot 6 to write on the sign to get people into the coffee shop.