I'd have to say the chemtrails over my house when I was a child. Nothing like sitting in a backyard full of grass while staring up at an inbound from the west coast.

Also all the flights I took as a child. Whenever anyone asks me why I chose to become a Pilot, I reply asking if they ever remembered the vacations they took when they were younger. They usually reply yes and ask me how that has anything to do with their question.. Then I tell em that I barely remember any of the trips or vacations, but I remember every single flight I've ever taken, the Airplanes I flew on, the Airlines I flew with, and the Pilots who flew us.