Movement
I love watching how things move. I often stand on the ends of train platforms when I'm in the subway and stand, watching the train come in, the people inside going past very fast, then slower. With each passing second, you can see more details. The reverse is interesting too: the people inside are clearly visible as the doors close, then as the train starts, they move past slowly, then becoming a blur.
If this wasn't something I'd done since I was a child, I'd think I was losing my mind. When I was small, I'd lay on the ground and play with doors, moving them back and forth, just watching how they move.