Rain upon a tin roof whilst lying in bed.
Also, the varying smells/feels of each season. I love the day when you get up and you suddenly feel like it is truly Autumn, or Summer - that the season has finally changed. It's something to do with the temperature, the different smells in the air, and how the sun is shining that day. I can't really explain it, maybe I'm imagining it, but I love it anyway.