Good question, and there are plenty of answers!
It will be of no great surprise to learn that the planets and stars are a source of tremendous curiosity to me. Looking up at the sky on a clear night, knowing those pinpricks of light are distant stars, leaves me awestruck. There is so much beautiful, and wonder, to be had in the cosmos.
The opposite end of the scale is quite fascinating too. Atoms, and their myriad component parts, are a source of much intrigue, but don’t trust atoms, they make up everything (groan).
There’s plenty of other stuff. One subject I don’t tend to bring up is that of carnivorous plants. Venus flytraps are the most famous of these, but the evolution of numerous varieties of pitcher plants, to say nothing of their mechanisms, is another greatly interesting curiosity.
The history of the railways is another, on-going source of wonder for me. It has been since I was a kid, as have trains in general. I don’t know why, but I’ve always found them… soothing, I guess?