As a writing exercise, it's fun to use a prompt from somewhere, and then freeflow.
To freeflow just set a timer for as little as two minutes, or as much more as you like, and then write without stopping until the timer goes off. Write all the gibberish you like. The important thing is not to stop. Don't stop because of spelling mistakes, bad grammar, you changed your mind or forgot how to spell a word. For this exercise you just have to keep going.
If you find that you've got the germ of a good idea here, you can fix it up later.
Pictures are one thing you can use as a prompt. Here's a link to a picture over at Flickr of a grazing horse to start with.