The Sunny 16 Rule

On a sunny day: set aperture to f/16, shutter speed to 1/ISO. That's it.

ISO 400: f/16, 1/400 (use 1/500)

ISO 200: f/16, 1/200

ISO 100: f/16, 1/100

From f/16 in direct sun, open up as conditions change:

Slight overcast: f/11

Overcast / open shade: f/8

Heavy overcast: f/5.6

Indoors / deep shade: f/4 or wider

This works because the sun's brightness is consistent. Film photographers used this before light meters. It is accurate enough to get you in the right ballpark without touching your camera's meter — which is useful when you want to keep your eye on the scene.

The point: knowing this means you can set exposure before you raise the camera. Less fumbling, more shooting.