Go to a place in public, like a coffee shop or a park, where you can sit and free write (pen to paper without stopping) for at least ten minutes. Free write about the environment around you and the people around you. Ideally, do this three times over the next three days or over the course of a week. Notice who the regulars are, notice patterns of behavior, site-specific jargon, recurring situations. Who and what are you drawn to? Develop a scene based on your observations.
Now, you don't have to go to the same spot more than once if you don't have the time or inclination. You can sit and write and absorb the place and people around you like a sponge and discover your scene from there. Pick two people around you and throw them into a scene together - not literally, on paper or on your lap top - and start exploring how they might encounter one another. Find the people or things that most people aren't noticing - the quiet, but efficient busboy, the homeless guy sifting through trash outside, the spilt sugar on the condiment station, etc, and build your scene around that.
You can write about anything anywhere, but if you are writing about where you are at, it connects you to the space and instead of coming up with something to write, you're more like taking dictation from the universe.