I was in a skyway over Hennepin Ave on a rainy day, trying to come up with photos of cars on wet streets, headlights and taillights reflecting in puddles, that sort of thing. And getting nothing interesting.
The auto-focus sometimes locked onto the raindrops on the glass right in front of me instead of the street below, which was annoying until I checked one on the LCD and thought – hey I like this. More than the photo I actually had in mind.
Looking at it later, I wanted the drops to appear bigger and the DOF shallow. So I went back in a couple of days (still raining) with a 135mm lens that would focus close and had nice bokeh. At f5.6 the lights on the cars and signs became discs and blurs of color. And then one crazy drop ran down from top to bottom. Some postprocessing, to enhance the curve and balance the brightness of different colors, and I had it.
The lesson for me was that maybe the thing to do is just go somewhere interesting and shoot, without being too committed to a specific goal.