I've had great conversations with strangers about whether aliens exist, or whether they can be made of sand, or rock, or gas. The intelligent people know how to link that to other things - temperatures, evolution, whether planets or cities can be sentient, the definition of consciousness. You can talk about things like souls or ghosts too.
You can also open up fixed topics that everyone has an opinion about. Politics, religion/life, economics, and investing are nice. Everyone intelligent has some kind of investment strategy. A lot of people at least try to figure out religion at some point. A lot of intelligence is understanding that you could be wrong, and dealing with that. If someone straight up says, "Communism bad, end of story," without trying to understand why communism is bad, they're likely not very smart.
The best answer, but someone that "understand" why communism is bad might still be wrong (and by consequence, not been that smart, just tricked one level deeper)
You can also open up fixed topics that everyone has an opinion about. Politics, religion/life, economics, and investing are nice. Everyone intelligent has some kind of investment strategy. A lot of people at least try to figure out religion at some point. A lot of intelligence is understanding that you could be wrong, and dealing with that. If someone straight up says, "Communism bad, end of story," without trying to understand why communism is bad, they're likely not very smart.