When I used to teach writing, I would tell my students to "vomit on the page." Not a wonderfully picturesque metaphor, I know, but it works. Just sit down and start writing whatever ridiculous notions you can think of. If you need to, talk as you write. That sometimes helps me (much to the consternation of everyone else in the coffee shop).
When all else fails, make a list of topics you are curious about. Do a quick, 5-minute research project on each one, and see where that leads you.