Take a five-minute walk, then make something using whatever materials are available where you've ended up. Leave it there for someone else to discover, but be sure to document it first.
I decided to tackle this one on my morning walk to work. I took a slightly different, more scenic route than usual, eventually stopping by a small man-made (frozen!) pond close to the canal. Carefully plucking maple seeds off the snow, i cleaned them up and rearranged them in patterns. Here, i should mention that it was -26C windchill at the time – meaning that my fingers were frozen by the time i was done!
Also of note: i was practically in someone's backyard while doing this, and at one point i looked up at the house and noticed someone inside looking at me. I walked by the 'scene of the art' the next morning, and was surprised to see the seeds perfectly undisturbed. This was especially surprising as the pond acts as a skating rink and dog park.