I find I can sometimes walk for miles/hours without seeing anything I want to photograph. But as soon as I take out the camera and start composing something, anything, I start seeing endless possibilities. So I have a rule that if I have not found anything to photograph in the first 15 minutes of an outing, I stop, take out the camera, and start framing whatever happens to be there. I'll usually end up finding something compelling enough to photograph. I think the camera acts like an on/off button sometime; in the bag = off; on the tripod = on.