Just general discussion and chit-chat. No topic.
Me: "Hello. How are you doing? Nice weather, shame about all the cats."
Me: "Cats. You know, in the atmosphere…"
Since this is supposed to be casual, refraining from the following three and one-half subjects would be appreciated:
0.5) Work (but that mostly applies to people who work together)
Those topics spin almost any conversation into an endlessly recursive monoculture of debate. There are others, true, (I’m looking at you, grammar!) but you get the idea.
Keep it casual, kids! Random is a pretty good idea, too.